feat(d3): implement quiver-basic

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

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+// anyplot.ai
+// quiver-basic: Basic Quiver Plot
+// Library: d3 7.9.0 | JavaScript 22.23.1
+// Quality: 92/100 | Created: 2026-07-24
+//# anyplot-orientation: square
+
+const t = window.ANYPLOT_TOKENS;
+const { width, height } = window.ANYPLOT_SIZE;
+
+// --- Data: ocean-current vortex sampled on a uniform buoy grid -------------
+// A rotating eddy (u = -y, v = x) with Gaussian decay from the eddy core,
+// a classic pattern for coastal/oceanographic current surveys.
+const gridSize = 15;
+const domainExtent = 3; // km from the eddy core
+const points = [];
+for (let i = 0; i < gridSize; i++) {
+  for (let j = 0; j < gridSize; j++) {
+    const x = -domainExtent + (2 * domainExtent * i) / (gridSize - 1);
+    const y = -domainExtent + (2 * domainExtent * j) / (gridSize - 1);
+    const decay = Math.exp(-(x * x + y * y) / 6);
+    const u = -y * decay; // eastward component (cm/s)
+    const v = x * decay; // northward component (cm/s)
+    points.push({ x, y, u, v, mag: Math.sqrt(u * u + v * v) });
+  }
+}
+const maxMag = d3.max(points, (d) => d.mag);
+
+// --- Layout: force a square plot area so vector angles render undistorted -
+const margin = { top: 140, right: 90, bottom: 110, left: 110 };
+const availW = width - margin.left - margin.right;
+const availH = height - margin.top - margin.bottom;
+const side = Math.min(availW, availH);
+const offsetX = margin.left + (availW - side) / 2;
+const offsetY = margin.top + (availH - side) / 2;
+
+// --- Scales -----------------------------------------------------------------
+const xScale = d3.scaleLinear().domain([-domainExtent, domainExtent]).range([0, side]);
+const yScale = d3.scaleLinear().domain([-domainExtent, domainExtent]).range([side, 0]);
+const cellSize = side / (gridSize - 1);
+const pxPerUnit = (cellSize * 0.85) / maxMag; // arrow length scaled to fit the cell
+const color = d3.scaleSequential(d3.interpolateRgbBasis(t.seq)).domain([0, maxMag]);
+
+// --- SVG mount ----------------------------------------------------------------
+const svg = d3.select("#container").append("svg").attr("width", width).attr("height", height);
+const g = svg.append("g").attr("transform", `translate(${offsetX},${offsetY})`);
+
+// --- Axes ---------------------------------------------------------------------
+const xAxis = g
+  .append("g")
+  .attr("transform", `translate(0,${side})`)
+  .call(d3.axisBottom(xScale).ticks(7).tickFormat((d) => `${d}`));
+const yAxis = g.append("g").call(d3.axisLeft(yScale).ticks(7).tickFormat((d) => `${d}`));
+for (const ax of [xAxis, yAxis]) {
+  ax.selectAll("text").attr("fill", t.inkSoft).style("font-size", "14px");
+  ax.selectAll("line").attr("stroke", t.grid);
+  ax.select(".domain").attr("stroke", t.inkSoft).attr("stroke-width", 0.75).attr("stroke-opacity", 0.5);
+}
+
+// faint reference grid so arrow positions read against the coordinate frame
+g.selectAll("line.gridx")
+  .data(xScale.ticks(7))
+  .join("line")
+  .attr("class", "gridx")
+  .attr("x1", (d) => xScale(d))
+  .attr("x2", (d) => xScale(d))
+  .attr("y1", 0)
+  .attr("y2", side)
+  .attr("stroke", t.grid)
+  .attr("stroke-width", 1);
+g.selectAll("line.gridy")
+  .data(yScale.ticks(7))
+  .join("line")
+  .attr("class", "gridy")
+  .attr("x1", 0)
+  .attr("x2", side)
+  .attr("y1", (d) => yScale(d))
+  .attr("y2", (d) => yScale(d))
+  .attr("stroke", t.grid)
+  .attr("stroke-width", 1);
+
+// subtle radial guide marking the eddy core's characteristic decay radius
+const coreRadiusPx = xScale(Math.sqrt(6)) - xScale(0);
+g.append("circle")
+  .attr("cx", xScale(0))
+  .attr("cy", yScale(0))
+  .attr("r", coreRadiusPx)
+  .attr("fill", "none")
+  .attr("stroke", t.inkSoft)
+  .attr("stroke-width", 1)
+  .attr("stroke-dasharray", "4,4")
+  .attr("stroke-opacity", 0.35);
+
+// --- Arrows (line + solid triangular head, colored by current speed) --------
+const headAngle = Math.PI / 7;
+const headLen = Math.max(7, cellSize * 0.2);
+const minShaftLen = Math.max(headLen * 1.5, cellSize * 0.3); // keep near-zero vectors legible
+
+const arrows = g.selectAll("g.arrow").data(points).join("g").attr("class", "arrow");
+arrows.each(function (d) {
+  const x0 = xScale(d.x);
+  const y0 = yScale(d.y);
+  const fill = color(d.mag);
+
+  if (d.mag < 1e-9) {
+    // exact stagnation point at the eddy core: direction undefined, mark with a dot
+    d3.select(this).append("circle").attr("cx", x0).attr("cy", y0).attr("r", 3).attr("fill", fill);
+    return;
+  }
+
+  const lenPx = Math.max(d.mag * pxPerUnit, minShaftLen);
+  const dxPx = (d.u / d.mag) * lenPx;
+  const dyPx = (-d.v / d.mag) * lenPx; // screen y grows downward, data y grows upward
+  const x1 = x0 + dxPx;
+  const y1 = y0 + dyPx;
+  const angle = Math.atan2(dyPx, dxPx);
+
+  const hx1 = x1 - headLen * Math.cos(angle - headAngle);
+  const hy1 = y1 - headLen * Math.sin(angle - headAngle);
+  const hx2 = x1 - headLen * Math.cos(angle + headAngle);
+  const hy2 = y1 - headLen * Math.sin(angle + headAngle);
+
+  d3.select(this)
+    .append("line")
+    .attr("x1", x0)
+    .attr("y1", y0)
+    .attr("x2", x1)
+    .attr("y2", y1)
+    .attr("stroke", fill)
+    .attr("stroke-width", 2.5)
+    .attr("stroke-linecap", "round");
+
+  d3.select(this)
+    .append("path")
+    .attr("d", `M${x1},${y1} L${hx1},${hy1} L${hx2},${hy2} Z`)
+    .attr("fill", fill);
+});
+
+// --- Axis labels --------------------------------------------------------------
+g.append("text")
+  .attr("x", side / 2)
+  .attr("y", side + 70)
+  .attr("text-anchor", "middle")
+  .attr("fill", t.ink)
+  .style("font-size", "18px")
+  .text("Distance east of eddy core (km)");
+
+g.append("text")
+  .attr("transform", `translate(${-78},${side / 2}) rotate(-90)`)
+  .attr("text-anchor", "middle")
+  .attr("fill", t.ink)
+  .style("font-size", "18px")
+  .text("Distance north of eddy core (km)");
+
+// --- Speed legend (color encodes current magnitude) --------------------------
+const legendW = 180;
+const legendH = 14;
+const legendX = offsetX + side - legendW;
+const legendY = offsetY - 60;
+
+const gradientId = "quiver-speed-gradient";
+const defs = svg.append("defs");
+const gradient = defs
+  .append("linearGradient")
+  .attr("id", gradientId)
+  .attr("x1", "0%")
+  .attr("x2", "100%");
+gradient.append("stop").attr("offset", "0%").attr("stop-color", t.seq[0]);
+gradient.append("stop").attr("offset", "100%").attr("stop-color", t.seq[1]);
+
+svg
+  .append("rect")
+  .attr("x", legendX)
+  .attr("y", legendY)
+  .attr("width", legendW)
+  .attr("height", legendH)
+  .attr("fill", `url(#${gradientId})`);
+
+svg
+  .append("text")
+  .attr("x", legendX)
+  .attr("y", legendY - 10)
+  .attr("text-anchor", "start")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "13px")
+  .text("Current speed");
+
+svg
+  .append("text")
+  .attr("x", legendX)
+  .attr("y", legendY + legendH + 20)
+  .attr("text-anchor", "start")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "13px")
+  .text("0 cm/s");
+
+svg
+  .append("text")
+  .attr("x", legendX + legendW)
+  .attr("y", legendY + legendH + 20)
+  .attr("text-anchor", "end")
+  .attr("fill", t.inkSoft)
+  .style("font-size", "13px")
+  .text(`${maxMag.toFixed(1)} cm/s`);
+
+// --- Title ----------------------------------------------------------------
+svg
+  .append("text")
+  .attr("x", width / 2)
+  .attr("y", 60)
+  .attr("text-anchor", "middle")
+  .attr("fill", t.ink)
+  .style("font-size", "26px")
+  .style("font-weight", "600")
+  .text("quiver-basic · javascript · d3 · anyplot.ai");