Kentucky Population Density by mbostock

Created via the U.S. Atlas project as make topo/ky-counties.json, after modifying the Makefile to include population and area statistics. These per-county values are then used to compute the population density of each county.

topojson \
    --id-property=FIPS,STATE_FIPS \
    -p name=COUNTY,name=STATE,+population,area=+SQUARE_MIL \
    -e KY-counties-pop.csv \
    -- shp/ky/counties.shp shp/ky/states.shp

Colors by ColorBrewer.

index.html

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

svg {
  font: 10px sans-serif;
}

.caption {
  font-weight: bold;
}

.key path {
  display: none;
}

.key line {
  stroke: #000;
  shape-rendering: crispEdges;
}

.county {
  fill: #eee;
}

.county-border {
  fill: none;
  stroke: #fff;
}

.state-border {
  fill: none;
  stroke: #333;
}

</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 color = d3.scale.threshold()
    .domain([30, 60, 120, 360])
    .range(["#ffffcc","#c2e699","#78c679","#31a354","#006837"]);

// A position encoding for the key only.
var x = d3.scale.linear()
    .domain([0, 390])
    .range([0, 240]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .tickSize(13)
    .tickValues(color.domain());

// Kentucky State Plane, Single Zone
// ftp://kygeonet.ky.gov/kygeodata/standards/Ky_StatePlane.pdf
var projection = d3.geo.conicConformal()
    .rotate([85 + 45 / 60, -36 - 20 / 60])
    .parallels([37 + 05 / 60, 38 + 40 / 60])
    .scale(7090)
    .center([-0.04940, 1.50406])
    .translate([width / 2, height / 2]);

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

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

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

g.selectAll("rect")
    .data(color.range().map(function(d, i) {
      return {
        x0: i ? x(color.domain()[i - 1]) : x.range()[0],
        x1: i < color.domain().length ? x(color.domain()[i]) : x.range()[1],
        z: d
      };
    }))
  .enter().append("rect")
    .attr("height", 8)
    .attr("x", function(d) { return d.x0; })
    .attr("width", function(d) { return d.x1 - d.x0; })
    .style("fill", function(d) { return d.z; });

g.call(xAxis).append("text")
    .attr("class", "caption")
    .attr("y", -6)
    .text("Population per square mile");

d3.json("ky-counties.json", function(error, ky) {
  if (error) throw error;

  var counties = topojson.feature(ky, ky.objects.counties);

  svg.append("g")
      .attr("class", "county")
    .selectAll("path")
      .data(counties.features)
    .enter().append("path")
      .attr("d", path)
      .style("fill", function(d) { return color(d.density = d.properties.population / d.properties.area); })
    .append("title")
      .text(function(d) { return d.properties.name + ": " + d.density.toFixed(0) + "/mi.²"; });

  svg.append("path")
      .datum(topojson.mesh(ky, ky.objects.counties, function(a, b) { return a !== b; }))
      .attr("class", "county-border")
      .attr("d", path);

  svg.append("path")
      .datum(topojson.mesh(ky, ky.objects.counties, function(a, b) { return a === b; }))
      .attr("class", "state-border")
      .attr("d", path);
});

</script>