california population per square mile

<!DOCTYPE html>
<meta charset="utf-8">
<link href='https://fonts.googleapis.com/css?family=Cardo:400,400italic,700' rel='stylesheet' type='text/css'>
<link href="https://fonts.googleapis.com/css?family=Bungee+Hairline" rel="stylesheet">
<style>

body {

background: #f5f5f5;

}

.caption {

font-family: 'Cardo';
  font-size: 18px;
  font-style: italic; 
  font-weight: 400; 
}

.tick {

font-family: 'Cardo';
  font-size: 18px;
  font-style: italic; 
  font-weight: 400; 
}

.counties {
  stroke: #fff;
  stroke-width: .25px;
}

.states {
  stroke: #fff;
  stroke-linejoin: round;
}

.axis {
  font-family: Futura;
  font-size: 11px;
}

</style>
<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>

// my style applied to a mbostock example: https://bl.ocks.org/mbostock/39b34968ad5eab65de1d7da81f78bb27

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.schemePuOr[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")
    .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;

var defs = svg.append("defs");

//Filter for the outside glow
  var filter = defs.append("filter")
    .attr("id","glow");
filter.append("feGaussianBlur")
    .attr("stdDeviation","3.5")
    .attr("result","coloredBlur");
var feMerge = filter.append("feMerge");
feMerge.append("feMergeNode")
    .attr("in","coloredBlur");
feMerge.append("feMergeNode")
    .attr("in","SourceGraphic");

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

svg.append("path")
      .datum(topojson.feature(topology, topology.objects.counties))
      .attr("fill", "none")
      .attr("stroke", "#000")
      .attr("stroke-width", ".25px")
      .attr("stroke-opacity", 0.3)
      .attr("d", path);

d3.selectAll("path")
    .style("filter", "url(#glow)");
});

</script>

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

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

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