diff --git a/app/web/main.py b/app/web/main.py
index 006143b..6cae799 100644
--- a/app/web/main.py
+++ b/app/web/main.py
@@ -13,6 +13,7 @@ from __future__ import annotations
 import importlib.util
 import json
 import os
+from functools import lru_cache
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Tuple
 
@@ -21,6 +22,13 @@ from fastapi import FastAPI, Form, Request
 from fastapi.responses import HTMLResponse, JSONResponse
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
+from sklearn.datasets import (
+    make_blobs,
+    make_classification,
+    make_gaussian_quantiles,
+    make_s_curve,
+    make_swiss_roll,
+)
 
 
 # ---------------------------------------------------------------------------
@@ -35,11 +43,137 @@ FIGS_DIR.mkdir(parents=True, exist_ok=True)
 PREFECT_API = os.environ.get("PREFECT_API_URL", "http://localhost:4200/api")
 DEPLOYMENT_NAME = "embedding-flow/embedding-flow"
 
-GENERATOR_OPTIONS = [
-    ("sklearn.datasets.make_s_curve", "make_s_curve"),
-    ("sklearn.datasets.make_swiss_roll", "make_swiss_roll"),
-    ("sklearn.datasets.make_blobs", "make_blobs"),
-]
+
+# ---------------------------------------------------------------------------
+# Dataset catalogue
+# ---------------------------------------------------------------------------
+# Metadata for the /data.json endpoint consumed by the dataset picker, and
+# for server-side lookup when the picker posts its selection back. kwargs
+# must carry n_features=3 for generators that aren't already 3-D, since
+# they'll be forwarded verbatim to the Prefect flow's generator_kwargs.
+
+DATASET_PREVIEW_N = 5000
+DATASET_PREVIEW_SEED = 0
+
+DATASET_META: Dict[str, Dict[str, Any]] = {
+    "s_curve": {
+        "name": "S-Curve",
+        "path": "sklearn.datasets.make_s_curve",
+        "kwargs": {},
+        "description": (
+            "A 2-D manifold warped into R³. Continuous label encodes position "
+            "along the curve — a good test of whether a reducer unrolls the "
+            "sheet without tearing."
+        ),
+        "kind": "continuous",
+    },
+    "swiss_roll": {
+        "name": "Swiss Roll",
+        "path": "sklearn.datasets.make_swiss_roll",
+        "kwargs": {},
+        "description": (
+            "A rolled-up plane. The canonical hard case for linear methods: "
+            "PCA collapses the spiral, non-linear methods should recover the "
+            "unroll."
+        ),
+        "kind": "continuous",
+    },
+    "swiss_roll_hole": {
+        "name": "Swiss Roll (hole)",
+        "path": "sklearn.datasets.make_swiss_roll",
+        "kwargs": {"hole": True},
+        "description": (
+            "Swiss roll with a rectangular hole punched through. Same manifold, "
+            "non-trivial topology — a faithful unroll should preserve the hole "
+            "rather than smearing it closed."
+        ),
+        "kind": "continuous",
+    },
+    "blobs": {
+        "name": "Gaussian Blobs",
+        "path": "sklearn.datasets.make_blobs",
+        "kwargs": {"n_features": 3, "centers": 5, "cluster_std": 1.0},
+        "description": (
+            "Five isotropic Gaussian clusters in R³. Discrete class labels. "
+            "Tests whether a reducer preserves cluster separation when "
+            "projected to 2-D."
+        ),
+        "kind": "categorical",
+    },
+    "gaussian_quantiles": {
+        "name": "Gaussian Quantiles",
+        "path": "sklearn.datasets.make_gaussian_quantiles",
+        "kwargs": {"n_features": 3, "n_classes": 4},
+        "description": (
+            "Concentric Gaussian shells in R³; class = which shell. Classes "
+            "are linearly inseparable by construction — PCA collapses them, "
+            "kernel and manifold methods have a chance."
+        ),
+        "kind": "categorical",
+    },
+    "classification": {
+        "name": "Hypercube Clusters",
+        "path": "sklearn.datasets.make_classification",
+        "kwargs": {
+            "n_features": 3,
+            "n_informative": 3,
+            "n_redundant": 0,
+            "n_repeated": 0,
+            "n_classes": 4,
+            "n_clusters_per_class": 2,
+            "class_sep": 1.5,
+        },
+        "description": (
+            "Four classes, two sub-clusters each, placed at hypercube vertices "
+            "with informative noise. A denser discrete test than blobs — "
+            "within-class bimodality stresses cluster-preserving reducers."
+        ),
+        "kind": "categorical",
+    },
+}
+
+
+@lru_cache(maxsize=1)
+def _dataset_previews() -> Dict[str, Dict[str, Any]]:
+    """Attach freshly-generated points+labels to the catalogue for the picker."""
+    N, SEED = DATASET_PREVIEW_N, DATASET_PREVIEW_SEED
+    s, sl = make_s_curve(n_samples=N, noise=0.03, random_state=SEED)
+    sr, srl = make_swiss_roll(n_samples=N, noise=0.15, random_state=SEED)
+    srh, srhl = make_swiss_roll(n_samples=N, noise=0.15, hole=True, random_state=SEED)
+    b, bl = make_blobs(
+        n_samples=N, n_features=3, centers=5, cluster_std=1.0, random_state=SEED
+    )
+    gq, gql = make_gaussian_quantiles(
+        n_samples=N, n_features=3, n_classes=4, random_state=SEED
+    )
+    cls, clsl = make_classification(
+        n_samples=N,
+        n_features=3,
+        n_informative=3,
+        n_redundant=0,
+        n_repeated=0,
+        n_classes=4,
+        n_clusters_per_class=2,
+        class_sep=1.5,
+        random_state=SEED,
+    )
+    samples = {
+        "s_curve": (s, sl),
+        "swiss_roll": (sr, srl),
+        "swiss_roll_hole": (srh, srhl),
+        "blobs": (b, bl),
+        "gaussian_quantiles": (gq, gql),
+        "classification": (cls, clsl),
+    }
+    out: Dict[str, Dict[str, Any]] = {}
+    for key, meta in DATASET_META.items():
+        pts, labels = samples[key]
+        out[key] = {
+            **meta,
+            "points": pts.tolist(),
+            "labels": labels.tolist(),
+        }
+    return out
 
 
 # ---------------------------------------------------------------------------
@@ -394,7 +528,6 @@ async def index(request: Request) -> HTMLResponse:
             "reducers": reducers,
             "default_reducer": default_reducer,
             "default_spec": default_spec,
-            "generators": GENERATOR_OPTIONS,
             "runs": views,
             "deployment_id": dep_id,
             "prefect_api": PREFECT_API,
@@ -402,6 +535,11 @@ async def index(request: Request) -> HTMLResponse:
     )
 
 
+@app.get("/data.json")
+async def data_json() -> JSONResponse:
+    return JSONResponse(_dataset_previews())
+
+
 @app.get("/reducer-form", response_class=HTMLResponse)
 async def reducer_form(request: Request, name: str) -> HTMLResponse:
     spec = REDUCERS.get(name)
@@ -436,7 +574,24 @@ async def submit(request: Request) -> HTMLResponse:
             status_code=400,
         )
 
-    # Data params
+    # Dataset came from the picker via dataset_id; fall back to explicit
+    # generator_path / generator_kwargs if a client posts those directly.
+    dataset_id = data.get("dataset_id") or ""
+    if dataset_id and dataset_id in DATASET_META:
+        meta = DATASET_META[dataset_id]
+        generator_path = meta["path"]
+        generator_kwargs = dict(meta["kwargs"])
+    else:
+        generator_path = data.get("generator_path") or "sklearn.datasets.make_s_curve"
+        raw_kwargs = data.get("generator_kwargs") or ""
+        try:
+            generator_kwargs = json.loads(raw_kwargs) if raw_kwargs else {}
+        except json.JSONDecodeError as e:
+            return HTMLResponse(
+                f"<div class='flash err'>bad generator_kwargs JSON: {e}</div>",
+                status_code=400,
+            )
+
     try:
         num_points = int(data.get("num_points", "5000") or 5000)
         num_snapshots = int(data.get("num_snapshots", "48") or 48)
@@ -446,14 +601,9 @@ async def submit(request: Request) -> HTMLResponse:
         return HTMLResponse(
             f"<div class='flash err'>bad numeric input: {e}</div>", status_code=400
         )
-    generator_path = data.get("generator_path") or "sklearn.datasets.make_s_curve"
 
     embed_args = build_embed_args(reducer, data)
 
-    generator_kwargs: Dict[str, Any] = {}
-    if generator_path.endswith("make_blobs"):
-        generator_kwargs["n_features"] = 3
-
     parameters: Dict[str, Any] = {
         "num_points": num_points,
         "num_snapshots": num_snapshots,
diff --git a/app/web/static/dataset-picker.js b/app/web/static/dataset-picker.js
new file mode 100644
index 0000000..a0588e4
--- /dev/null
+++ b/app/web/static/dataset-picker.js
@@ -0,0 +1,367 @@
+// Dataset picker — ported from app/demo/index.html. Renders the six sklearn
+// previews into cards, streams jittered random walks, and writes the current
+// selection into hidden form inputs so the main <form> can submit it to the
+// Prefect flow.
+import * as THREE from 'three';
+import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
+
+// Trajectories are precomputed at the max cycle length. Toggling n-frames
+// truncates (12 is a prefix of 24 is a prefix of 48) so the same walk is
+// reused — no reroll on toggle, and the per-frame pulse stays consistent.
+const MAX_FRAMES = 48;
+
+const CATEGORICAL_HEX = [
+  '#1f4e5f', '#c97b3f', '#8b5a9f', '#5a8560',
+  '#c74a5e', '#6b7d8f', '#b89f51', '#4a6fa5',
+];
+const CATEGORICAL = CATEGORICAL_HEX.map(h => new THREE.Color(h));
+
+function rampContinuous(t) {
+  const hue = (1 - t) * 215 + t * 28;
+  const sat = 0.62;
+  const lit = 0.50 + (t - 0.5) * 0.08;
+  return new THREE.Color().setHSL(hue / 360, sat, lit);
+}
+
+function normalize(points) {
+  const n = points.length;
+  let mx = 0, my = 0, mz = 0;
+  for (const p of points) { mx += p[0]; my += p[1]; mz += p[2]; }
+  mx /= n; my /= n; mz /= n;
+
+  // p95 of per-point max-coord magnitude — robust to distribution tails
+  // (gaussian_quantiles / classification otherwise shrink to a fraction of
+  // the viewport under pure max-abs normalization).
+  const perPoint = new Float64Array(n);
+  for (let i = 0; i < n; i++) {
+    const p = points[i];
+    const a = Math.abs(p[0] - mx);
+    const b = Math.abs(p[1] - my);
+    const c = Math.abs(p[2] - mz);
+    perPoint[i] = a > b ? (a > c ? a : c) : (b > c ? b : c);
+  }
+  const sorted = Array.from(perPoint).sort((a, b) => a - b);
+  const scale = Math.max(sorted[Math.floor(n * 0.95)], 1e-9);
+
+  const out = new Float32Array(n * 3);
+  for (let i = 0; i < n; i++) {
+    out[i*3]   = (points[i][0] - mx) / scale;
+    out[i*3+1] = (points[i][1] - my) / scale;
+    out[i*3+2] = (points[i][2] - mz) / scale;
+  }
+  return { positions: out };
+}
+
+function buildColors(labels, kind) {
+  const n = labels.length;
+  const colors = new Float32Array(n * 3);
+  if (kind === 'categorical') {
+    for (let i = 0; i < n; i++) {
+      const c = CATEGORICAL[labels[i] % CATEGORICAL.length];
+      colors[i*3] = c.r; colors[i*3+1] = c.g; colors[i*3+2] = c.b;
+    }
+  } else {
+    let lo = Infinity, hi = -Infinity;
+    for (const v of labels) { if (v < lo) lo = v; if (v > hi) hi = v; }
+    const range = (hi - lo) || 1;
+    for (let i = 0; i < n; i++) {
+      const c = rampContinuous((labels[i] - lo) / range);
+      colors[i*3] = c.r; colors[i*3+1] = c.g; colors[i*3+2] = c.b;
+    }
+  }
+  return colors;
+}
+
+function createScene(container, dataset) {
+  const { positions: basePositions } = normalize(dataset.points);
+  const colors = buildColors(dataset.labels, dataset.kind);
+  const positions = new Float32Array(basePositions);
+
+  const geometry = new THREE.BufferGeometry();
+  geometry.setAttribute('position', new THREE.BufferAttribute(positions, 3));
+  geometry.setAttribute('color', new THREE.BufferAttribute(colors, 3));
+
+  const material = new THREE.PointsMaterial({
+    size: 2.1,
+    sizeAttenuation: false,
+    vertexColors: true,
+    transparent: true,
+    opacity: 0.92,
+  });
+
+  const scene = new THREE.Scene();
+  scene.background = new THREE.Color(0xf2eee4);
+  scene.add(new THREE.Points(geometry, material));
+
+  const camera = new THREE.PerspectiveCamera(42, 1, 0.1, 100);
+  camera.position.set(2.6, 1.9, 2.6);
+  camera.lookAt(0, 0, 0);
+
+  const renderer = new THREE.WebGLRenderer({ antialias: true, alpha: false });
+  renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2));
+  container.appendChild(renderer.domElement);
+
+  const controls = new OrbitControls(camera, renderer.domElement);
+  controls.enableDamping = true;
+  controls.dampingFactor = 0.08;
+  controls.enablePan = false;
+  controls.autoRotate = true;
+  controls.autoRotateSpeed = 0.55;
+  controls.minDistance = 1.5;
+  controls.maxDistance = 6;
+
+  return {
+    scene, camera, renderer, controls, container, geometry,
+    basePositions,
+    trajectories: null,
+    numFrames: 0,
+    snapshotMs: 1000 / 12,
+    // holdMs pads the end of each cycle at rest, so frame-0-as-rest is
+    // actually visible (otherwise it's zero-duration).
+    holdMs: 200,
+    // Shared across scenes — applyF() resets all of them together so the
+    // three previews stay in lockstep through n-frames toggles.
+    cycleStartMs: 0,
+  };
+}
+
+function buildTrajectories(s, numFrames) {
+  const n = s.basePositions.length;
+  const buf = new Float32Array(numFrames * n);
+  for (let f = 1; f < numFrames; f++) {
+    const prev = (f - 1) * n;
+    const cur = f * n;
+    for (let j = 0; j < n; j += 2) {
+      const u1 = Math.random() || 1e-12;
+      const u2 = Math.random();
+      const mag = Math.sqrt(-2.0 * Math.log(u1));
+      buf[cur + j] = buf[prev + j] + mag * Math.cos(2 * Math.PI * u2);
+      if (j + 1 < n) buf[cur + j + 1] = buf[prev + j + 1] + mag * Math.sin(2 * Math.PI * u2);
+    }
+  }
+  s.trajectories = buf;
+  s.numFrames = numFrames;
+}
+
+function sizeScene(s) {
+  const rect = s.container.getBoundingClientRect();
+  const size = Math.max(1, Math.floor(rect.width));
+  s.renderer.setSize(size, size);
+  s.camera.aspect = 1;
+  s.camera.updateProjectionMatrix();
+}
+
+async function main() {
+  const gallery = document.getElementById('gallery');
+  let data;
+  try {
+    const res = await fetch('/data.json');
+    data = await res.json();
+  } catch (err) {
+    gallery.innerHTML =
+      `<div class="picker-loading">failed to load /data.json — ${err}</div>`;
+    return;
+  }
+
+  gallery.innerHTML = '';
+  const scenes = [];
+  const order = Object.entries(data);
+  let selectedId = null;
+
+  // Hidden form inputs the main <form> will submit.
+  const hidden = {
+    datasetId: document.getElementById('dataset_id'),
+    numPoints: document.getElementById('num_points'),
+    numSnapshots: document.getElementById('num_snapshots'),
+    jitterScale: document.getElementById('jitter_scale'),
+  };
+
+  const pickerDetails = document.getElementById('picker');
+  const summaryPath = document.getElementById('picker-summary-path');
+  const selectedPath = document.getElementById('selected-path');
+  const continueBtn = document.getElementById('continue-btn');
+
+  const vizToScene = new WeakMap();
+  const sizeObserver = new ResizeObserver((entries) => {
+    for (const entry of entries) {
+      const s = vizToScene.get(entry.target);
+      if (s) sizeScene(s);
+    }
+  });
+
+  order.forEach(([id, ds], i) => {
+    const card = document.createElement('div');
+    card.className = 'card';
+    card.dataset.id = id;
+    card.innerHTML = `
+      <div class="viz">
+        <span class="fig-label">Fig. 1.${i + 1}</span>
+        <span class="key-hint">[${i + 1}]</span>
+        <span class="controls-hint">drag · scroll</span>
+      </div>
+      <div class="card-body">
+        <div class="card-label">
+          <span class="dot"></span>
+          <span>${ds.name}</span>
+        </div>
+        <div class="card-path">${ds.path}</div>
+        <div class="card-desc">${ds.description}</div>
+      </div>
+    `;
+    gallery.appendChild(card);
+
+    const viz = card.querySelector('.viz');
+    const s = createScene(viz, ds);
+    buildTrajectories(s, MAX_FRAMES);
+    scenes.push(s);
+    vizToScene.set(viz, s);
+    sizeObserver.observe(viz);
+
+    s.controls.addEventListener('start', () => { s.controls.autoRotate = false; });
+
+    card.addEventListener('click', () => selectCard(id, card, ds));
+  });
+
+  function selectCard(id, card, ds) {
+    document.querySelectorAll('#gallery .card').forEach(c => c.classList.remove('selected'));
+    card.classList.add('selected');
+    selectedId = id;
+    selectedPath.textContent = ds.path;
+    hidden.datasetId.value = id;
+    updateContinue();
+  }
+
+  function updateContinue() {
+    continueBtn.disabled = !selectedId;
+    continueBtn.title = selectedId ? '' : 'pick a dataset first';
+  }
+
+  const slider = document.getElementById('n-slider');
+  const nValue = document.getElementById('n-value');
+  function applyN(n) {
+    nValue.textContent = n.toLocaleString();
+    hidden.numPoints.value = String(n);
+    for (const s of scenes) {
+      const cap = s.geometry.attributes.position.count;
+      s.geometry.setDrawRange(0, Math.min(n, cap));
+    }
+  }
+  slider.addEventListener('input', (e) => applyN(parseInt(e.target.value, 10)));
+  applyN(parseInt(slider.value, 10));
+
+  let jitterScale = 0;
+  const jInputs = document.querySelectorAll('input[name="j"]');
+  function applyJ(v) {
+    jitterScale = v;
+    hidden.jitterScale.value = String(v);
+  }
+  jInputs.forEach(input => {
+    input.addEventListener('change', (e) => applyJ(parseFloat(e.target.value)));
+  });
+  applyJ(parseFloat(document.querySelector('input[name="j"]:checked').value));
+
+  // n frames: truncates the precomputed walk. Prefix-stable (12 ⊂ 24 ⊂ 48),
+  // so toggling changes cycle length without rerolling. cycleStartMs is
+  // shared so all cards animate in lockstep.
+  const fInputs = document.querySelectorAll('input[name="f"]');
+  function applyF(n) {
+    hidden.numSnapshots.value = String(n);
+    const start = performance.now();
+    for (const s of scenes) {
+      s.numFrames = n;
+      s.cycleStartMs = start;
+    }
+  }
+  fInputs.forEach(input => {
+    input.addEventListener('change', (e) => applyF(parseInt(e.target.value, 10)));
+  });
+  applyF(parseInt(document.querySelector('input[name="f"]:checked').value, 10));
+
+  function selectByIndex(idx, { scroll = true } = {}) {
+    const entry = order[idx];
+    if (!entry) return;
+    const [id, ds] = entry;
+    const card = gallery.children[idx];
+    if (!card) return;
+    selectCard(id, card, ds);
+    if (scroll) card.scrollIntoView({ behavior: 'smooth', inline: 'nearest', block: 'nearest' });
+  }
+  document.addEventListener('keydown', (e) => {
+    if (!pickerDetails.open) return;
+    if (e.target.tagName === 'INPUT' || e.target.tagName === 'TEXTAREA') return;
+    if (/^[1-9]$/.test(e.key)) {
+      selectByIndex(parseInt(e.key, 10) - 1);
+      return;
+    }
+    if (e.key === 'ArrowRight' || e.key === 'ArrowLeft') {
+      e.preventDefault();
+      const currentIdx = order.findIndex(([id]) => id === selectedId);
+      const n = order.length;
+      const nextIdx = e.key === 'ArrowRight'
+        ? (currentIdx < 0 ? 0 : Math.min(currentIdx + 1, n - 1))
+        : (currentIdx < 0 ? n - 1 : Math.max(currentIdx - 1, 0));
+      selectByIndex(nextIdx);
+    }
+  });
+
+  continueBtn.addEventListener('click', () => {
+    if (!selectedId) return;
+    const ds = data[selectedId];
+    summaryPath.textContent = ds.path;
+    pickerDetails.open = false;
+  });
+
+  function tick() {
+    requestAnimationFrame(tick);
+    // When the picker is collapsed the canvases are display:none inside a
+    // closed <details>; rects are zero. Skip the per-frame work.
+    if (!pickerDetails.open) return;
+    const now = performance.now();
+    for (const s of scenes) {
+      const N = s.numFrames;
+      const n = s.basePositions.length;
+      const walkMs = N * s.snapshotMs;
+      const cycleMs = walkMs + s.holdMs;
+      const elapsed = ((now - s.cycleStartMs) % cycleMs + cycleMs) % cycleMs;
+
+      const total = n / 3;
+      const drawCount = s.geometry.drawRange.count;
+      const visibleN = Number.isFinite(drawCount) ? Math.min(drawCount, total) : total;
+      const limit = visibleN * 3;
+      const pos = s.geometry.attributes.position.array;
+      const base = s.basePositions;
+
+      if (elapsed >= walkMs) {
+        for (let i = 0; i < limit; i++) pos[i] = base[i];
+      } else {
+        const frameF = elapsed / s.snapshotMs;
+        const frameIdx = Math.floor(frameF);
+        const interpT = frameF - frameIdx;
+        const nextIdx = (frameIdx + 1) % N;
+        const aOff = frameIdx * n;
+        const bOff = nextIdx * n;
+        const tr = s.trajectories;
+        const scale = jitterScale;
+        const u = 1 - interpT;
+        for (let i = 0; i < limit; i++) {
+          pos[i] = base[i] + (tr[aOff + i] * u + tr[bOff + i] * interpT) * scale;
+        }
+      }
+      s.geometry.attributes.position.needsUpdate = true;
+
+      s.controls.update();
+      s.renderer.render(s.scene, s.camera);
+    }
+  }
+  tick();
+
+  // Reopening the picker after it's been closed: canvases may have been
+  // laid out at zero size while hidden. Re-measure on toggle.
+  pickerDetails.addEventListener('toggle', () => {
+    if (pickerDetails.open) {
+      for (const s of scenes) sizeScene(s);
+    }
+  });
+}
+
+main();
diff --git a/app/web/static/style.css b/app/web/static/style.css
index 706443e..f717a93 100644
--- a/app/web/static/style.css
+++ b/app/web/static/style.css
@@ -513,3 +513,351 @@ button.submit:disabled { background: var(--faint); border-color: var(--faint); c
 }
 .htmx-request .htmx-indicator { opacity: 1; }
 .htmx-request.htmx-indicator  { opacity: 1; }
+
+/* ---------- dataset picker (§ 1) -------------------------------------- */
+/* Collapsible <details> below the masthead; expands to a gallery of 3D
+   dataset previews, collapses to a one-line summary chip once confirmed. */
+
+.dataset-picker {
+  --picker-panel: #f2eee4;
+  --picker-hair:  #d8d3c6;
+  border-bottom: 1px solid var(--rule);
+  background: var(--page);
+}
+.dataset-picker > summary {
+  list-style: none;
+  cursor: pointer;
+  padding: 0.9rem 2.2rem;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  gap: 1rem;
+  user-select: none;
+}
+.dataset-picker > summary::-webkit-details-marker { display: none; }
+.dataset-picker > summary:hover { background: var(--accent-tint); }
+.dataset-picker .picker-meta {
+  display: flex;
+  align-items: baseline;
+  gap: 1rem;
+  flex-wrap: wrap;
+  min-width: 0;
+}
+.dataset-picker .section-number {
+  font-family: var(--mono);
+  font-size: 0.78rem;
+  font-weight: 600;
+  color: var(--accent);
+  letter-spacing: 0;
+}
+.dataset-picker .picker-title {
+  font-family: var(--serif);
+  font-size: 1rem;
+  color: var(--ink);
+  font-style: italic;
+}
+.dataset-picker .picker-selection {
+  font-size: 0.78rem;
+  color: var(--mute);
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0.5rem;
+}
+.dataset-picker .picker-selection .lbl {
+  font-family: var(--mono);
+  font-size: 0.68rem;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--faint);
+}
+.dataset-picker .picker-selection code {
+  font-family: var(--mono);
+  font-size: 0.78rem;
+  color: var(--accent);
+}
+.dataset-picker .picker-toggle {
+  font-family: var(--mono);
+  font-size: 0.74rem;
+  color: var(--faint);
+  letter-spacing: 0;
+}
+.dataset-picker .picker-toggle::before { content: "[ edit ]"; }
+.dataset-picker[open] .picker-toggle::before { content: "[ collapse ]"; color: var(--accent); }
+
+.dataset-picker .picker-body {
+  padding: 0.4rem 2.2rem 1.6rem;
+  max-width: 1440px;
+  margin: 0 auto;
+}
+.dataset-picker .lede {
+  color: var(--mute);
+  font-family: var(--serif);
+  font-style: italic;
+  font-size: 0.88rem;
+  max-width: 62ch;
+  margin: 0.2rem 0 0.9rem;
+}
+.dataset-picker .lede kbd {
+  font-family: var(--mono);
+  font-size: 0.72rem;
+  background: var(--panel);
+  border: 1px solid var(--rule);
+  padding: 0 5px;
+  margin: 0 1px;
+  color: var(--ink);
+}
+
+.picker-controls {
+  display: grid;
+  grid-template-columns: auto 1fr auto;
+  align-items: center;
+  column-gap: 1rem;
+  row-gap: 0.55rem;
+  padding: 0.75rem 0;
+  border-top: 1px solid var(--rule);
+  border-bottom: 1px solid var(--rule);
+  margin-bottom: 1.1rem;
+}
+.picker-controls .ctl-label {
+  font-family: var(--mono);
+  font-size: 0.68rem;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--mute);
+}
+.picker-controls .ctl-value {
+  font-family: var(--mono);
+  font-size: 0.82rem;
+  color: var(--accent);
+  min-width: 3.5rem;
+  text-align: right;
+  font-variant-numeric: tabular-nums;
+}
+.picker-controls input[type="range"] {
+  width: 100%;
+  accent-color: var(--accent);
+  height: 4px;
+}
+.picker-controls .segmented {
+  grid-column: 2 / -1;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+}
+.picker-controls .segmented label {
+  font-family: var(--mono);
+  font-size: 0.78rem;
+  color: var(--mute);
+  cursor: pointer;
+  padding: 3px 2px 4px;
+  border-bottom: 1px solid transparent;
+  transition: color 120ms ease, border-color 120ms ease;
+  user-select: none;
+  font-variant-numeric: tabular-nums;
+  position: relative;
+}
+.picker-controls .segmented label:hover { color: var(--ink); }
+.picker-controls .segmented label:has(input:checked) {
+  color: var(--accent);
+  border-bottom-color: var(--accent);
+}
+.picker-controls .segmented label:has(input:focus-visible) {
+  outline: 1px solid var(--accent);
+  outline-offset: 2px;
+}
+.picker-controls .segmented input[type="radio"] {
+  position: absolute;
+  opacity: 0;
+  width: 1px; height: 1px;
+  margin: 0;
+}
+
+.dataset-picker .gallery {
+  display: flex;
+  gap: 1.15rem;
+  overflow-x: auto;
+  overflow-y: hidden;
+  scroll-snap-type: x mandatory;
+  scroll-behavior: smooth;
+  -webkit-overflow-scrolling: touch;
+  padding: 2px 2px 0.85rem;
+  margin: 0 -2px 1.1rem;
+  scrollbar-width: thin;
+  scrollbar-color: var(--rule-2) transparent;
+}
+.dataset-picker .gallery::-webkit-scrollbar { height: 6px; }
+.dataset-picker .gallery::-webkit-scrollbar-track { background: transparent; }
+.dataset-picker .gallery::-webkit-scrollbar-thumb {
+  background: var(--rule-2);
+  border-radius: 3px;
+}
+.dataset-picker .gallery::-webkit-scrollbar-thumb:hover { background: var(--mute); }
+
+.dataset-picker .picker-loading {
+  padding: 3rem 0;
+  text-align: center;
+  color: var(--mute);
+  font-family: var(--mono);
+  font-size: 0.82rem;
+}
+
+.dataset-picker .card {
+  flex: 0 0 240px;
+  /* Prevent long .card-path from forcing the card wider than its flex-basis. */
+  min-width: 0;
+  scroll-snap-align: start;
+  border: 1px solid var(--rule);
+  background: var(--picker-panel);
+  cursor: pointer;
+  display: flex;
+  flex-direction: column;
+  transition: border-color 120ms ease, box-shadow 120ms ease;
+}
+.dataset-picker .card:hover { border-color: var(--rule-2); }
+.dataset-picker .card.selected {
+  border-color: var(--accent);
+  box-shadow: 0 0 0 1px var(--accent);
+}
+.dataset-picker .viz {
+  aspect-ratio: 1 / 1;
+  position: relative;
+  overflow: hidden;
+}
+.dataset-picker .viz canvas {
+  position: absolute;
+  inset: 0;
+  display: block;
+  width: 100% !important;
+  height: 100% !important;
+}
+.dataset-picker .fig-label {
+  position: absolute;
+  top: 8px;
+  left: 10px;
+  font-family: var(--mono);
+  font-size: 0.62rem;
+  color: var(--mute);
+  letter-spacing: 0.05em;
+  text-transform: uppercase;
+  pointer-events: none;
+}
+.dataset-picker .key-hint {
+  position: absolute;
+  top: 6px;
+  right: 8px;
+  font-family: var(--mono);
+  font-size: 0.7rem;
+  color: var(--mute);
+  background: rgba(250, 250, 247, 0.85);
+  border: 1px solid var(--rule);
+  padding: 0 5px;
+  pointer-events: none;
+}
+.dataset-picker .card.selected .key-hint {
+  color: var(--accent);
+  border-color: var(--accent);
+}
+.dataset-picker .controls-hint {
+  position: absolute;
+  bottom: 6px;
+  right: 8px;
+  font-family: var(--mono);
+  font-size: 0.62rem;
+  color: var(--mute);
+  opacity: 0;
+  transition: opacity 150ms ease;
+  pointer-events: none;
+}
+.dataset-picker .card:hover .controls-hint { opacity: 0.75; }
+.dataset-picker .card-body {
+  padding: 0.75rem 0.9rem 0.9rem;
+  border-top: 1px solid var(--rule);
+  background: var(--page);
+  flex: 1;
+}
+.dataset-picker .card-label {
+  font-family: var(--sans);
+  font-weight: 500;
+  font-size: 0.9rem;
+  margin-bottom: 2px;
+  display: flex;
+  align-items: baseline;
+  gap: 0.5rem;
+}
+.dataset-picker .card-label .dot {
+  display: inline-block;
+  width: 7px; height: 7px;
+  border-radius: 50%;
+  background: transparent;
+  border: 1px solid var(--rule-2);
+}
+.dataset-picker .card.selected .card-label .dot {
+  background: var(--accent);
+  border-color: var(--accent);
+}
+.dataset-picker .card-path {
+  font-family: var(--mono);
+  font-size: 0.68rem;
+  color: var(--mute);
+  margin-bottom: 0.4rem;
+  word-break: break-all;
+}
+.dataset-picker .card-desc {
+  font-family: var(--serif);
+  font-size: 0.82rem;
+  color: #4a4a4a;
+  line-height: 1.5;
+}
+
+.picker-footer {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding-top: 0.85rem;
+  border-top: 1px solid var(--rule);
+  gap: 1.2rem;
+  flex-wrap: wrap;
+}
+.picker-footer .selection {
+  font-size: 0.82rem;
+  color: var(--mute);
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0.5rem;
+}
+.picker-footer .selection .lbl {
+  font-family: var(--mono);
+  font-size: 0.68rem;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+}
+.picker-footer .selection code {
+  font-family: var(--mono);
+  font-size: 0.82rem;
+  color: var(--accent);
+}
+.picker-footer .continue {
+  background: var(--accent);
+  color: var(--page);
+  border: 1px solid var(--accent);
+  padding: 0.45rem 1rem;
+  font-family: var(--sans);
+  font-size: 0.82rem;
+  font-weight: 600;
+  letter-spacing: 0.04em;
+  cursor: pointer;
+  border-radius: 1px;
+  transition: background 120ms ease;
+}
+.picker-footer .continue:not(:disabled):hover { background: #143642; }
+.picker-footer .continue:disabled {
+  background: var(--faint);
+  border-color: var(--faint);
+  color: var(--page);
+  cursor: not-allowed;
+}
+
+@media (max-width: 940px) {
+  .dataset-picker > summary { padding: 0.9rem 1.2rem; }
+  .dataset-picker .picker-body { padding: 0.4rem 1.2rem 1.4rem; }
+}
diff --git a/app/web/templates/index.html b/app/web/templates/index.html
index 65134f7..0e11bb3 100644
--- a/app/web/templates/index.html
+++ b/app/web/templates/index.html
@@ -6,6 +6,14 @@
   <title>embedding notebook — web1</title>
   <link rel="stylesheet" href="/static/style.css" />
   <script src="https://unpkg.com/htmx.org@2.0.4"></script>
+  <script type="importmap">
+  {
+    "imports": {
+      "three": "https://unpkg.com/three@0.160.0/build/three.module.js",
+      "three/addons/": "https://unpkg.com/three@0.160.0/examples/jsm/"
+    }
+  }
+  </script>
   <link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16'%3E%3Ccircle cx='8' cy='8' r='3' fill='%231f4e5f'/%3E%3C/svg%3E" />
 </head>
 <body>
@@ -21,6 +29,59 @@
   </div>
 </header>
 
+<details class="dataset-picker" id="picker" open>
+  <summary>
+    <span class="picker-meta">
+      <span class="section-number">§ 1</span>
+      <span class="picker-title">input dataset</span>
+      <span class="picker-selection">
+        <span class="lbl">generator</span>
+        <code id="picker-summary-path">—</code>
+      </span>
+    </span>
+    <span class="picker-toggle" aria-hidden="true"></span>
+  </summary>
+
+  <div class="picker-body">
+    <p class="lede">
+      Six candidate generators for the embedding pipeline. Drag to rotate, scroll to zoom,
+      <kbd>←</kbd>&nbsp;<kbd>→</kbd> or <kbd>1</kbd>&thinsp;&hellip;&thinsp;<kbd>6</kbd> to select.
+    </p>
+
+    <div class="picker-controls">
+      <label class="ctl-label" for="n-slider">n samples</label>
+      <input type="range" id="n-slider" min="100" max="5000" step="100" value="500">
+      <span class="ctl-value" id="n-value">500</span>
+
+      <span class="ctl-label">noise σ</span>
+      <div class="segmented" role="radiogroup" aria-label="noise σ">
+        <label><input type="radio" name="j" value="0.001"><span>0.001</span></label>
+        <label><input type="radio" name="j" value="0.005" checked><span>0.005</span></label>
+        <label><input type="radio" name="j" value="0.01"><span>0.010</span></label>
+      </div>
+
+      <span class="ctl-label">n frames</span>
+      <div class="segmented" role="radiogroup" aria-label="number of frames">
+        <label><input type="radio" name="f" value="12"><span>12</span></label>
+        <label><input type="radio" name="f" value="24" checked><span>24</span></label>
+        <label><input type="radio" name="f" value="48"><span>48</span></label>
+      </div>
+    </div>
+
+    <div class="gallery" id="gallery">
+      <div class="picker-loading">loading samples&hellip;</div>
+    </div>
+
+    <div class="picker-footer">
+      <div class="selection">
+        <span class="lbl">generator</span>
+        <code id="selected-path">—</code>
+      </div>
+      <button type="button" class="continue" id="continue-btn" disabled>Continue &rarr;</button>
+    </div>
+  </div>
+</details>
+
 <main>
 
   <!-- ==================== LEFT: parameter notebook ==================== -->
@@ -34,10 +95,17 @@
       hx-indicator="#busy"
     >
 
-      <!-- §1 reducer -->
+      <!-- Picker-driven hidden fields. Values are written by dataset-picker.js. -->
+      <input type="hidden" name="dataset_id" id="dataset_id" value="" />
+      <input type="hidden" name="num_points" id="num_points" value="500" />
+      <input type="hidden" name="num_snapshots" id="num_snapshots" value="24" />
+      <input type="hidden" name="jitter_scale" id="jitter_scale" value="0.005" />
+      <input type="hidden" name="seed" id="seed" value="42" />
+
+      <!-- §2 reducer -->
       <div class="section">
         <div class="section-label">
-          <span>§ 1 &nbsp; reducer</span><span class="ordinal">method</span>
+          <span>§ 2 &nbsp; reducer</span><span class="ordinal">method</span>
         </div>
         <p class="lead">Dimensionality reduction applied to each snapshot. Only reducers whose Python package is importable are shown.</p>
 
@@ -63,53 +131,16 @@
         </ul>
       </div>
 
-      <!-- §2 reducer params -->
+      <!-- §3 reducer params -->
       <div class="section">
         <div class="section-label">
-          <span>§ 2 &nbsp; parameters</span><span class="ordinal">kwargs</span>
+          <span>§ 3 &nbsp; parameters</span><span class="ordinal">kwargs</span>
         </div>
         <div id="reducer-params">
           {% include "_reducer_form.html" with context %}
         </div>
       </div>
 
-      <!-- §3 data -->
-      <div class="section">
-        <div class="section-label">
-          <span>§ 3 &nbsp; data &amp; drift</span><span class="ordinal">sampling</span>
-        </div>
-
-        <div class="form-grid">
-          <label for="generator_path">
-            generator
-            <span class="hint">data-generating surface</span>
-          </label>
-          <select name="generator_path" id="generator_path">
-            {% for path, short in generators %}
-              <option value="{{ path }}" {% if path == 'sklearn.datasets.make_s_curve' %}selected{% endif %}>{{ short }}</option>
-            {% endfor %}
-          </select>
-
-          <label for="num_points">n<sub>points</sub></label>
-          <input type="number" id="num_points" name="num_points" value="5000" min="100" step="100" />
-
-          <label for="num_snapshots">n<sub>snapshots</sub></label>
-          <input type="number" id="num_snapshots" name="num_snapshots" value="48" min="2" step="1" />
-
-          <label for="jitter_scale">
-            jitter scale
-            <span class="hint">std of per-step Gaussian drift</span>
-          </label>
-          <input type="number" id="jitter_scale" name="jitter_scale" value="0.01" step="0.001" min="0" />
-
-          <label for="seed">
-            jitter seed
-            <span class="hint">seeds only the drift simulation — the embedder's seed is in §2 (advanced).</span>
-          </label>
-          <input type="number" id="seed" name="seed" value="42" step="1" />
-        </div>
-      </div>
-
       <div class="actions">
         <button type="submit" class="submit">submit run</button>
         <span id="busy" class="htmx-indicator">dispatching&hellip;</span>
@@ -145,9 +176,11 @@
 </main>
 
 <footer class="colophon">
-  <span><span class="k">web1</span> · scientific instrument · port 8001</span>
+  <span><span class="k">web</span> · scientific instrument · port 8001</span>
   <span>fastapi · htmx · no build step</span>
 </footer>
 
+<script type="module" src="/static/dataset-picker.js"></script>
+
 </body>
 </html>