Bivariate choropleth

const labels = ["low", "", "high"];
const n = Math.floor(Math.sqrt(colors.length));
const x = d3.scaleQuantile(Array.from(data, (d) => d.diabetes), d3.range(n));
const y = d3.scaleQuantile(Array.from(data, (d) => d.obesity), d3.range(n));
const index = d3.index(data, (d) => d.county);
const path = d3.geoPath();

function color(value) {
  if (!value) return "#ccc";
  const {diabetes: a, obesity: b} = value;
  return colors[y(b) + x(a) * n];
}

function format(value) {
  if (!value) return "N/A";
  const {diabetes: a, obesity: b} = value;
  return `${a}% Diabetes${labels[x(a)] && ` (${labels[x(a)]})`}
${b}% Obesity${labels[y(b)] && ` (${labels[y(b)]})`}`;
}

const svg = d3.create("svg")
    .attr("width", 975)
    .attr("height", 610)
    .attr("viewBox", [0, 0, 975, 610])
    .attr("style", "max-width: 100%; height: auto;");

svg.append(() => {
      const k = 24; // cell size
      const arrow = DOM.uid();
      return htl.svg`<g transform="translate(870, 450)" style="font: 12px var(--sans-serif);">
  <g transform="translate(-${k * n / 2},-${k * n / 2}) rotate(-45 ${k * n / 2},${k * n / 2})">
    <marker id="${arrow.id}" markerHeight=10 markerWidth=10 refX=6 refY=3 orient=auto>
      <path d="M0,0L9,3L0,6Z" />
    </marker>
    ${d3.cross(d3.range(n), d3.range(n)).map(([i, j]) => htl.svg`<rect width=${k} height=${k} x=${i * k} y=${(n - 1 - j) * k} fill=${colors[j * n + i]}>
      <title>Diabetes${labels[j] && ` (${labels[j]})`}
Obesity${labels[i] && ` (${labels[i]})`}</title>
    </rect>`)}
    <line marker-end="${arrow}" x1=0 x2=${n * k} y1=${n * k} y2=${n * k} stroke=black stroke-width=1.5 />
    <line marker-end="${arrow}" y2=0 y1=${n * k} stroke=black stroke-width=1.5 />
    <text font-weight="bold" dy="0.71em" transform="rotate(90) translate(${n / 2 * k},6)" text-anchor="middle">Diabetes</text>
    <text font-weight="bold" dy="0.71em" transform="translate(${n / 2 * k},${n * k + 6})" text-anchor="middle">Obesity</text>
  </g>
</g>`;
    });

svg.append("g")
  .selectAll("path")
  .data(topojson.feature(us, us.objects.counties).features)
  .join("path")
    .attr("fill", (d) => color(index.get(d.id)))
    .attr("d", path)
  .append("title")
    .text((d) => `${d.properties.name}, ${states.get(d.id.slice(0, 2)).name}
${format(index.get(d.id))}`);

svg.append("path")
    .datum(topojson.mesh(us, us.objects.states, (a, b) => a !== b))
    .attr("fill", "none")
    .attr("stroke", "white")
    .attr("stroke-linejoin", "round")
    .attr("d", path);

display(svg.node());

const data = FileAttachment("data/us-county-diabetes-obesity.csv").csv()
  .then((data) => {
    data.forEach((d) => {
      d.obesity = +d.obesity; // type as numeric
      d.diabetes = +d.diabetes;
    });
    return data;
  })
  .then(display);

const schemes = [
  {
    name: "RdBu",
    colors: [
      "#e8e8e8", "#e4acac", "#c85a5a",
      "#b0d5df", "#ad9ea5", "#985356",
      "#64acbe", "#627f8c", "#574249"
    ]
  },
  {
    name: "BuPu",
    colors: [
      "#e8e8e8", "#ace4e4", "#5ac8c8",
      "#dfb0d6", "#a5add3", "#5698b9",
      "#be64ac", "#8c62aa", "#3b4994"
    ]
  },
  {
    name: "GnBu",
    colors: [
      "#e8e8e8", "#b5c0da", "#6c83b5",
      "#b8d6be", "#90b2b3", "#567994",
      "#73ae80", "#5a9178", "#2a5a5b"
    ]
  },
  {
    name: "PuOr",
    colors: [
      "#e8e8e8", "#e4d9ac", "#c8b35a",
      "#cbb8d7", "#c8ada0", "#af8e53",
      "#9972af", "#976b82", "#804d36"
    ]
  }
];

const us = await FileAttachment("data/us-counties-albers-10m.json").json();
const states = display(new Map(us.objects.states.geometries.map((d) => [d.id, d.properties])));