US Infant Mortality by County - with legend

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

body {
    font-family: arial, sans;
    font-size: 14px;
    width: 960px;
    margin: 40px auto;
  }
  svg {
    border: 1px solid #ccc; 
  }

.states {
    stroke:#fff;
    fill:none;
  }

.counties {
    stroke: #fff;
    fill: green;
    stroke-width: .3px;
  }

.county-bubble {
    stroke:#fff;
    stroke-width: .3;
  }
</style>

<body>

</body>

var color = d3.scaleLinear()
	.range(["#d73027","#fc8d59","#fee08b","#ffffbf","#d9ef8b","#91cf60","#1a9850"])
	.domain([1300,1100,900,800,650,500,300]);

var radius = d3.scaleSqrt()
    .range([0,20])
    .domain([0,10000]);

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

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

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

//   console.log(deaths);
//   console.log(us);
//   console.log(topojson.feature(us, us.objects.counties).features);

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

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

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

svg.append("g")
      .attr("class", "counties")
    .selectAll("path")
      .data(subset)
    .enter().append("path")
      .attr("d", path)
      .style("fill", function(d) { 
        console.log(deathsByFips[d.id]["Crude Rate"]);
        return color(deathsByFips[d.id]["Crude Rate"])
      });

/*
  svg.append("g")
      .attr("class", "county-bubble-group")
    .selectAll(".county-bubble")
      .data(subset)
    .enter().append("circle")
      .attr("class", "county-bubble")
      .attr("d", path)
      .attr("transform", function(d) { return "translate(" + path.centroid(d) + ")"; })
      .attr("r", function(d) {
        return radius(deathsByFips[d.id].Deaths);
      })
      .style("fill", function(d) {
        return color(deathsByFips[d.id]["Crude Rate"]);
      })
      .on("mouseover", function(d) {
        console.log(deathsByFips[d.id]);
      });
      */

svg.append("path")
      .datum(topojson.mesh(us, us.objects.states, function(a, b) { return a.id !== b.id; }))
      .attr("class", "states")
      .attr("d", path);
svg.append("text")
	.attr("x", width - 130)
	.attr("y", height - 260)
	.text("Infant Mortality")
  
  var legend = svg.selectAll(".legend")
  .data(color.domain())
  .enter().append("g")
  .attr("transform", function(d,i){
    return "translate("+(width-130)+","+(height/2+16*i)+")";
  })
 legend.append("rect")
 .attr("width",12)
 .attr("height",12)
 .style("fill", function(d){
   return color(d);
 })

legend.append("text")
 	.attr("x",16)
 	.attr("y",7)
 	.attr("alighment-baseline","middle")
 	.text(function(d){
   	return d;
 });

}
</script>

Modified http://d3js.org/d3.v4.js to a secure url

Modified http://d3js.org/topojson.v1.min.js to a secure url

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

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