Canvas Choropleth by syntagmatic

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v4.js" charset="utf-8"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script type="text/javascript">
  
var width = 960,
    height = 500;

var color = d3.scaleThreshold()
  .range(["#fde0dd","#fcc5c0","#fa9fb5","#f768a1","#dd3497","#ae017e","#7a0177","#49006a"])
  .domain([250, 500, 750, 1000, 1250, 1500, 1750, 2000]);

// all these pixel ratio bits used to support full resolution on high-density (retina) devices
var devicePixelRatio = window.devicePixelRatio || 1;

var canvas = d3.select("body").append("canvas")
    .attr("width", width * devicePixelRatio)
    .attr("height", height * devicePixelRatio)
    .style("width", width + "px")
    .style("height", height + "px").node();

var context = canvas.getContext("2d");
context.scale(devicePixelRatio, devicePixelRatio);

var tooltip = d3.select("body")
    .append("div")
    .attr("class", "tooltip")
    .style("position", "absolute")
    .style("z-index", "10")
    .style("visibility", "hidden");

d3.queue()
    .defer(d3.tsv, "infants-1999-2015.txt")
    .defer(d3.json, "us.json")
    .await(ready);

function ready(error, deaths, us) {
  if (error) return console.warn(error);

  deathsByFips = {};
  deaths.forEach(function(d) {
    d.Deaths = +d.Deaths;
    d.Population = +d.Population;
    d["Crude Rate"] = +(d["Crude Rate"].replace(" (Unreliable)", ""));
    deathsByFips[+d["County Code"]] = d;
  });

  var path = d3.geoPath()
    .projection(d3.geoAlbersUsa())
    .context(context);

  var subset = topojson.feature(us, us.objects.counties).features.filter(function(d) {
    return d.id in deathsByFips;
  });

  context.strokeStyle = "#fff";
  context.lineWidth = 0.3;
  subset.forEach(function(d) {
    context.fillStyle = color(deathsByFips[d.id]["Crude Rate"]);
    context.beginPath();
    path(d);
    context.fill();
    context.stroke();
  });

  context.lineWidth = 1;
  path(topojson.mesh(us, us.objects.states, function(a, b) { return a.id !== b.id; }));
  context.stroke();
}

</script>