This choropleth uses a threshold scale for quantization, mapping arbitrary slices of a continuous domain to discrete values in the range. Unemployment rates ranging from 2 to 10% are quantized into different shades of purple.
var color = d3.scale.threshold()
.domain([0.02, 0.04, 0.06, 0.08, 0.10])
.range(["#f2f0f7", "#dadaeb", "#bcbddc", "#9e9ac8", "#756bb1", "#54278f"]);
<!DOCTYPE html>
<meta charset="utf-8">
<style>
.states {
fill: none;
stroke: #fff;
stroke-linejoin: round;
}
</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/queue.v1.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>
var width = 960,
height = 500;
var color = d3.scale.threshold()
.domain([0.02, 0.04, 0.06, 0.08, 0.10])
.range(["#f2f0f7", "#dadaeb", "#bcbddc", "#9e9ac8", "#756bb1", "#54278f"]);
var path = d3.geo.path();
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
queue()
.defer(d3.json, "/mbostock/raw/4090846/us.json")
.defer(d3.tsv, "unemployment.tsv")
.await(ready);
function ready(error, us, unemployment) {
if (error) throw error;
var rateById = {};
unemployment.forEach(function(d) { rateById[d.id] = +d.rate; });
svg.append("g")
.attr("class", "counties")
.selectAll("path")
.data(topojson.feature(us, us.objects.counties).features)
.enter().append("path")
.attr("d", path)
.style("fill", function(d) { return color(rateById[d.id]); });
svg.append("path")
.datum(topojson.mesh(us, us.objects.states, function(a, b) { return a.id !== b.id; }))
.attr("class", "states")
.attr("d", path);
}
</script>