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Oct 24, 2023
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Choropleth
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Also listed in…
Gallery
County Maps
d3-geo
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chart = {
const color = d3.scaleQuantize([1, 10], d3.schemeBlues[9]);
const path = d3.geoPath();
const format = d => `${d}%`;
const valuemap = new Map(data.map(d => [d.id, d.rate]));

// The counties feature collection is all U.S. counties, each with a
// five-digit FIPS identifier. The statemap lets us lookup the name of
// the state that contains a given county; a state’s two-digit identifier
// corresponds to the first two digits of its counties’ identifiers.
const counties = topojson.feature(us, us.objects.counties);
const states = topojson.feature(us, us.objects.states);
const statemap = new Map(states.features.map(d => [d.id, d]));

// The statemesh is just the internal borders between states, i.e.,
// everything but the coastlines and country borders. This avoids an
// additional stroke on the perimeter of the map, which would otherwise
// mask intricate features such as islands and inlets. (Try removing
// the last argument to topojson.mesh below to see the effect.)
const statemesh = topojson.mesh(us, us.objects.states, (a, b) => a !== 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("g")
.attr("transform", "translate(610,20)")
.append(() => Legend(color, {title: "Unemployment rate (%)", width: 260}));

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

return svg.node();
}
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data = (await FileAttachment("unemployment-x.csv").csv()).map((d) => (d.rate = +d.rate, d))
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us = FileAttachment("counties-albers-10m.json").json() // https://github.com/topojson/us-atlas
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import {Legend} from "@d3/color-legend"
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Plot.plot({
projection: "identity",
width: 975,
height: 610,
color: {scheme: "Blues", type: "quantize", n: 9, domain: [1, 10], label: "Unemployment rate (%)", legend: true},
marks: [
Plot.geo(topojson.feature(us, us.objects.counties), {fill: (map => d => map.get(d.id))(new Map(data.map(d => [d.id, d.rate])))}),
Plot.geo(topojson.mesh(us, us.objects.states, (a, b) => a !== b), {stroke: "white"})
]
})
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