Merged Choropleth

Source: American Community Survey, 2014 5-Year Estimate

A variation of my California population density map using topomerge to combine census tracts in the same density interval. The resulting TopoJSON file is substantially smaller (328K instead of 1.5M), renders faster, and looks better due to fewer antialiasing artifacts!

<!DOCTYPE html>
<svg width="960" height="1100"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>
<script>

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

var path = d3.geoPath();

var color = d3.scaleThreshold()
    .domain([1, 10, 50, 200, 500, 1000, 2000, 4000])
    .range(d3.schemeOrRd[9]);

var x = d3.scaleSqrt()
    .domain([0, 4500])
    .rangeRound([440, 950]);

var g = svg.append("g")
    .attr("class", "key")
    .attr("transform", "translate(0,40)");

g.selectAll("rect")
  .data(color.range().map(function(d) {
      d = color.invertExtent(d);
      if (d[0] == null) d[0] = x.domain()[0];
      if (d[1] == null) d[1] = x.domain()[1];
      return d;
    }))
  .enter().append("rect")
    .attr("height", 8)
    .attr("x", function(d) { return x(d[0]); })
    .attr("width", function(d) { return x(d[1]) - x(d[0]); })
    .attr("fill", function(d) { return color(d[0]); });

g.append("text")
    .attr("class", "caption")
    .attr("x", x.range()[0])
    .attr("y", -6)
    .attr("fill", "#000")
    .attr("text-anchor", "start")
    .attr("font-weight", "bold")
    .text("Population per square mile");

g.call(d3.axisBottom(x)
    .tickSize(13)
    .tickValues(color.domain()))
  .select(".domain")
    .remove();

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

svg.append("g")
    .selectAll("path")
    .data(topojson.feature(topology, topology.objects.tracts).features)
    .enter().append("path")
      .attr("fill", function(d) { return d3.schemeOrRd[9][d.id]; })
      .attr("d", path);

svg.append("path")
      .datum(topojson.feature(topology, topology.objects.counties))
      .attr("fill", "none")
      .attr("stroke", "#000")
      .attr("stroke-opacity", 0.3)
      .attr("d", path);
});

</script>

https://d3js.org/d3.v4.min.js

https://d3js.org/d3-scale-chromatic.v1.min.js

https://d3js.org/topojson.v2.min.js