feat(d3): implement dumbbell-basic (#9573)

## Implementation: `dumbbell-basic` - javascript/d3

Implements the **javascript/d3** version of `dumbbell-basic`.

**File:** `plots/dumbbell-basic/implementations/javascript/d3.js`

**Parent Issue:** #945

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/28481348050)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: 3 people

plots/dumbbell-basic/implementations/javascript/d3.js

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+// anyplot.ai
+// dumbbell-basic: Basic Dumbbell Chart
+// Library: d3 7.9.0 | JavaScript 22.23.1
+// Quality: 91/100 | Created: 2026-06-30
+
+const t = window.ANYPLOT_TOKENS;
+const { width, height } = window.ANYPLOT_SIZE;
+
+const margin = { top: 95, right: 130, bottom: 90, left: 175 };
+const iw = width - margin.left - margin.right;
+const ih = height - margin.top - margin.bottom;
+
+// Employee satisfaction scores (0–100) before and after flexible work policy
+const rawData = [
+  { dept: "Research",         before: 57, after: 79 },
+  { dept: "Engineering",      before: 72, after: 91 },
+  { dept: "Product",          before: 63, after: 81 },
+  { dept: "IT Support",       before: 65, after: 82 },
+  { dept: "Design",           before: 68, after: 84 },
+  { dept: "Operations",       before: 71, after: 83 },
+  { dept: "Marketing",        before: 60, after: 71 },
+  { dept: "Customer Success", before: 66, after: 76 },
+  { dept: "HR",               before: 75, after: 83 },
+  { dept: "Finance",          before: 62, after: 69 },
+];
+
+// Sort by improvement (largest gap at top)
+const data = rawData.slice().sort((a, b) => (b.after - b.before) - (a.after - a.before));
+data.forEach(d => { d.delta = d.after - d.before; });
+
+const svg = d3.select("#container").append("svg")
+  .attr("width", width)
+  .attr("height", height);
+
+// --- D3 SVG defs: linear gradient for connecting lines (green → lavender) ---
+// Defined in SVG user-space so position tracks the data scale exactly
+const defs = svg.append("defs");
+const grad = defs.append("linearGradient")
+  .attr("id", "line-grad")
+  .attr("gradientUnits", "userSpaceOnUse")
+  .attr("x1", margin.left)
+  .attr("x2", margin.left + iw)
+  .attr("y1", 0)
+  .attr("y2", 0);
+grad.append("stop").attr("offset", "0%")
+  .attr("stop-color", t.palette[0]).attr("stop-opacity", 0.65);
+grad.append("stop").attr("offset", "100%")
+  .attr("stop-color", t.palette[1]).attr("stop-opacity", 0.65);
+
+const g = svg.append("g").attr("transform", `translate(${margin.left},${margin.top})`);
+
+// Scales
+const x = d3.scaleLinear().domain([50, 100]).range([0, iw]);
+const y = d3.scaleBand()
+  .domain(data.map(d => d.dept))
+  .range([0, ih])
+  .padding(0.4);
+
+// Vertical gridlines
+g.selectAll(".vgrid")
+  .data(x.ticks(6))
+  .join("line")
+  .attr("x1", d => x(d))
+  .attr("x2", d => x(d))
+  .attr("y1", 0)
+  .attr("y2", ih)
+  .attr("stroke", t.grid)
+  .attr("stroke-width", 1);
+
+// X-axis — d3.format("d") produces clean integers with no decimal noise
+const xAxisG = g.append("g")
+  .attr("transform", `translate(0,${ih})`)
+  .call(d3.axisBottom(x).ticks(6).tickSize(0).tickFormat(d3.format("d")));
+
+xAxisG.select(".domain").attr("stroke", t.inkSoft);
+xAxisG.selectAll(".tick text")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "14px")
+  .attr("dy", "1.3em");
+xAxisG.selectAll(".tick line").remove();
+
+// Y-axis (no domain line — categories self-label)
+const yAxisG = g.append("g")
+  .call(d3.axisLeft(y).tickSize(0));
+
+yAxisG.select(".domain").remove();
+yAxisG.selectAll(".tick text")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "14px")
+  .attr("dx", "-0.6em");
+
+// Connecting lines — gradient stroke encodes before→after direction
+g.selectAll(".dumbbell-line")
+  .data(data)
+  .join("line")
+  .attr("x1", d => x(d.before))
+  .attr("x2", d => x(d.after))
+  .attr("y1", d => y(d.dept) + y.bandwidth() / 2)
+  .attr("y2", d => y(d.dept) + y.bandwidth() / 2)
+  .attr("stroke", "url(#line-grad)")
+  .attr("stroke-width", 3);
+
+// Before dots (Imprint palette[0] — brand green)
+g.selectAll(".dot-before")
+  .data(data)
+  .join("circle")
+  .attr("cx", d => x(d.before))
+  .attr("cy", d => y(d.dept) + y.bandwidth() / 2)
+  .attr("r", 10)
+  .attr("fill", t.palette[0])
+  .attr("stroke", t.pageBg)
+  .attr("stroke-width", 2);
+
+// After dots (Imprint palette[1] — lavender)
+g.selectAll(".dot-after")
+  .data(data)
+  .join("circle")
+  .attr("cx", d => x(d.after))
+  .attr("cy", d => y(d.dept) + y.bandwidth() / 2)
+  .attr("r", 10)
+  .attr("fill", t.palette[1])
+  .attr("stroke", t.pageBg)
+  .attr("stroke-width", 2);
+
+// Delta labels — d3.format("+d") shows signed improvement for each row
+const deltaFmt = d3.format("+d");
+g.selectAll(".delta-label")
+  .data(data)
+  .join("text")
+  .attr("x", iw + 14)
+  .attr("y", d => y(d.dept) + y.bandwidth() / 2 + 5)
+  .attr("fill", (d, i) => i < 2 ? t.palette[1] : t.inkSoft)
+  .style("font-size", "14px")
+  .style("font-weight", (d, i) => i < 2 ? "700" : "400")
+  .text(d => deltaFmt(d.delta) + " pts");
+
+// Callout annotations for top 2 improvements — placed in the gap below each band
+const annLabels = ["▲ Highest gain", "▲ Runner-up"];
+data.slice(0, 2).forEach((d, i) => {
+  const xMid = (x(d.before) + x(d.after)) / 2;
+  const yAnn = y(d.dept) + y.bandwidth() + 6;
+  g.append("text")
+    .attr("x", xMid)
+    .attr("y", yAnn + 10)
+    .attr("text-anchor", "middle")
+    .attr("fill", t.amber)
+    .style("font-size", "12px")
+    .style("font-weight", "600")
+    .text(annLabels[i]);
+});
+
+// X-axis label
+g.append("text")
+  .attr("x", iw / 2)
+  .attr("y", ih + 62)
+  .attr("text-anchor", "middle")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "14px")
+  .text("Satisfaction Score (0–100)");
+
+// Legend (horizontal, centered below title in top margin)
+const legendItems = [
+  { label: "Before Policy", color: t.palette[0] },
+  { label: "After Policy",  color: t.palette[1] },
+];
+const legendSpacing = 170;
+const legendStartX = (width - legendItems.length * legendSpacing) / 2;
+const legendY = 72;
+
+legendItems.forEach((item, i) => {
+  const lx = legendStartX + i * legendSpacing;
+  svg.append("circle")
+    .attr("cx", lx + 10).attr("cy", legendY)
+    .attr("r", 10)
+    .attr("fill", item.color)
+    .attr("stroke", t.pageBg)
+    .attr("stroke-width", 1.5);
+  svg.append("text")
+    .attr("x", lx + 26).attr("y", legendY + 5)
+    .attr("fill", t.inkSoft)
+    .style("font-size", "13px")
+    .text(item.label);
+});
+
+// Title — font-size scaled for long descriptive title
+const title = "Employee Satisfaction · dumbbell-basic · javascript · d3 · anyplot.ai";
+const titleFontSize = title.length > 67 ? Math.round(22 * 67 / title.length) : 22;
+
+svg.append("text")
+  .attr("x", width / 2)
+  .attr("y", 42)
+  .attr("text-anchor", "middle")
+  .attr("fill", t.ink)
+  .style("font-size", `${titleFontSize}px`)
+  .style("font-weight", "600")
+  .text(title);