feat(chartjs): implement residual-plot (#11591)

## Implementation: `residual-plot` - javascript/chartjs

Implements the **javascript/chartjs** version of `residual-plot`.

**File:** `plots/residual-plot/implementations/javascript/chartjs.js`

**Parent Issue:** #2332

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/33965741710)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/residual-plot/implementations/javascript/chartjs.js

@@ -0,0 +1,189 @@
+// anyplot.ai
+// residual-plot: Residual Plot
+// Library: chartjs 4.4.7 | JavaScript 22.23.2
+// Quality: 94/100 | Created: 2026-09-05
+
+const t = window.ANYPLOT_TOKENS;
+
+// --- Data (in-memory, deterministic LCG) ------------------------------------
+// Simulated linear-regression diagnostics: fitted house-price predictions
+// (in $1000s) vs. residuals, with mild heteroscedasticity (variance grows
+// with fitted value) so the fan-out pattern is visible.
+let seed = 42;
+function lcg() {
+  seed = (seed * 1664525 + 1013904223) % 4294967296;
+  return seed / 4294967296;
+}
+function gaussian() {
+  const u1 = 1 - lcg();
+  const u2 = lcg();
+  return Math.sqrt(-2 * Math.log(u1)) * Math.cos(2 * Math.PI * u2);
+}
+function hexToRgba(hex, alpha) {
+  const h = hex.replace("#", "");
+  const r = parseInt(h.substring(0, 2), 16);
+  const g = parseInt(h.substring(2, 4), 16);
+  const b = parseInt(h.substring(4, 6), 16);
+  return `rgba(${r}, ${g}, ${b}, ${alpha})`;
+}
+
+const n = 220;
+const fitted = [];
+const residuals = [];
+for (let i = 0; i < n; i++) {
+  const value = 150 + lcg() * 450; // fitted price, $150k-$600k
+  const noiseScale = 8 + (value - 150) * 0.05; // heteroscedastic spread
+  fitted.push(value);
+  residuals.push(gaussian() * noiseScale);
+}
+
+const mean = residuals.reduce((a, b) => a + b, 0) / n;
+const variance = residuals.reduce((a, b) => a + (b - mean) ** 2, 0) / n;
+const stdDev = Math.sqrt(variance);
+const threshold = 2 * stdDev;
+
+const normalPoints = [];
+const outlierPoints = [];
+for (let i = 0; i < n; i++) {
+  const point = { x: fitted[i], y: residuals[i] };
+  if (Math.abs(residuals[i]) > threshold) {
+    outlierPoints.push(point);
+  } else {
+    normalPoints.push(point);
+  }
+}
+
+const xMin = Math.min(...fitted);
+const xMax = Math.max(...fitted);
+
+// Rolling-mean smoothing trend (sorted by fitted value) to surface any
+// residual non-linearity — optional per spec, adds diagnostic value.
+const sortedIdx = fitted.map((_, i) => i).sort((a, b) => fitted[a] - fitted[b]);
+const sortedX = sortedIdx.map((i) => fitted[i]);
+const sortedY = sortedIdx.map((i) => residuals[i]);
+const windowSize = Math.max(15, Math.round(n * 0.12));
+const trendPoints = sortedX.map((x, i) => {
+  const lo = Math.max(0, i - Math.floor(windowSize / 2));
+  const hi = Math.min(n, i + Math.ceil(windowSize / 2));
+  const slice = sortedY.slice(lo, hi);
+  const avg = slice.reduce((a, b) => a + b, 0) / slice.length;
+  return { x, y: avg };
+});
+
+// --- Mount -------------------------------------------------------------------
+const canvas = document.createElement("canvas");
+document.getElementById("container").appendChild(canvas);
+
+// --- Chart ---------------------------------------------------------------------
+new Chart(canvas, {
+  type: "scatter",
+  data: {
+    datasets: [
+      {
+        label: "±2σ band",
+        data: [
+          { x: xMin, y: threshold },
+          { x: xMax, y: threshold },
+        ],
+        showLine: true,
+        borderColor: t.amber,
+        borderWidth: 1.5,
+        borderDash: [6, 4],
+        pointRadius: 0,
+        fill: "+2",
+        backgroundColor:
+          t.pageBg === "#1A1A17" ? "rgba(240,239,232,0.06)" : "rgba(26,26,23,0.04)",
+      },
+      {
+        label: "Zero reference",
+        data: [
+          { x: xMin, y: 0 },
+          { x: xMax, y: 0 },
+        ],
+        showLine: true,
+        borderColor: t.ink,
+        borderWidth: 2,
+        pointRadius: 0,
+      },
+      {
+        label: "−2σ band",
+        data: [
+          { x: xMin, y: -threshold },
+          { x: xMax, y: -threshold },
+        ],
+        showLine: true,
+        borderColor: t.amber,
+        borderWidth: 1.5,
+        borderDash: [6, 4],
+        pointRadius: 0,
+      },
+      {
+        label: "Residuals",
+        data: normalPoints,
+        backgroundColor: hexToRgba(t.palette[0], 0.7),
+        borderColor: t.pageBg,
+        borderWidth: 1,
+        pointRadius: 6,
+        pointHoverRadius: 7,
+      },
+      {
+        label: "Smoothed trend",
+        data: trendPoints,
+        showLine: true,
+        borderColor: t.palette[1],
+        borderWidth: 2,
+        borderDash: [3, 3],
+        pointRadius: 0,
+        fill: false,
+        tension: 0.3,
+      },
+      {
+        label: "Outliers (>2σ)",
+        data: outlierPoints,
+        backgroundColor: t.palette[4],
+        borderColor: t.pageBg,
+        borderWidth: 1,
+        pointRadius: 7,
+        pointStyle: "triangle",
+        pointHoverRadius: 8,
+      },
+    ],
+  },
+  options: {
+    responsive: true,
+    maintainAspectRatio: false,
+    animation: false,
+    plugins: {
+      title: {
+        display: true,
+        text: "residual-plot · javascript · chartjs · anyplot.ai",
+        color: t.ink,
+        font: { size: 22, weight: "500" },
+        padding: { bottom: 20 },
+      },
+      legend: {
+        labels: {
+          color: t.inkSoft,
+          font: { size: 14 },
+          filter: (item) => item.text !== "±2σ band" && item.text !== "−2σ band",
+        },
+      },
+      tooltip: { enabled: false },
+    },
+    scales: {
+      x: {
+        type: "linear",
+        title: { display: true, text: "Fitted Value ($1,000s)", color: t.ink, font: { size: 16 } },
+        ticks: { color: t.inkSoft, font: { size: 14 } },
+        grid: { color: t.grid },
+        border: { color: t.inkSoft },
+      },
+      y: {
+        title: { display: true, text: "Residual ($1,000s)", color: t.ink, font: { size: 16 } },
+        ticks: { color: t.inkSoft, font: { size: 14 } },
+        grid: { color: t.grid },
+        border: { color: t.inkSoft },
+      },
+    },
+  },
+});