Projected Choropleth

This variation of the unemployment choropleth uses topojson --projection to create projected TopoJSON with the d3.geo.albersUsa projection.

This example also uses the -e argument to join the county geometries with a TSV file of unemployment rates, simplifying the implementation and eliminating the need to load multiple data files.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.counties {
  fill: none;
}

.state-borders {
  fill: none;
  stroke: #fff;
  stroke-linejoin: round;
  stroke-linecap: round;
}

.q0-9 { fill:rgb(247,251,255); }
.q1-9 { fill:rgb(222,235,247); }
.q2-9 { fill:rgb(198,219,239); }
.q3-9 { fill:rgb(158,202,225); }
.q4-9 { fill:rgb(107,174,214); }
.q5-9 { fill:rgb(66,146,198); }
.q6-9 { fill:rgb(33,113,181); }
.q7-9 { fill:rgb(8,81,156); }
.q8-9 { fill:rgb(8,48,107); }

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 500;

var quantize = d3.scale.quantize()
    .domain([0, .15])
    .range(d3.range(9).map(function(i) { return "q" + i + "-9"; }));

var path = d3.geo.path()
    .projection(null);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("us.json", function(error, us) {
  if (error) throw error;

svg.append("g")
      .attr("class", "counties")
    .selectAll("path")
      .data(topojson.feature(us, us.objects.counties).features)
    .enter().append("path")
      .attr("class", function(d) { return quantize(d.properties.rate); })
      .attr("d", path);

svg.append("path")
      .datum(topojson.mesh(us, us.objects.counties, function(a, b) { return a.id / 1000 ^ b.id / 1000; }))
      .attr("class", "state-borders")
      .attr("d", path);
});

</script>

https://d3js.org/d3.v3.min.js

https://d3js.org/topojson.v1.min.js