nvTopo

<!DOCTYPE html>
<svg width="1100" height="1500"></svg>
<style>
  .hidden {
        display: none;
    }
    div.tooltip {
        color: #fff;
        background-color: DarkGoldenRod;
        padding: .5em;
     		font-family: Arial;
      	font-size: 12px;
      	font-weight: bolder;
        border-radius: 2px;
        opacity: 0.7;
        position: absolute;
    }
</style>
<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.schemeYlOrRd[9]);

var x = d3.scaleSqrt()
    .domain([0, 4500])
    .rangeRound([440, 1050]);
 var tooltip = d3.select('body').append('div')
            .attr('class', 'hidden tooltip');

var g = svg.append("g")
    .attr("class", "key")
    .attr("transform", "translate(-50,825)");

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("nv-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 color(d.properties.density); })
      .attr("d", path)
    	.attr("y",400)
			  .on('mousemove', function(d) {
                    var mouse = d3.mouse(svg.node()).map(function(d) {
                        return parseInt(d);
                    });
                    tooltip.classed('hidden', false)
                        .attr('style', 'left:' + (mouse[0] + 15) +
                                'px; top:' + (mouse[1] - 25) + 'px')
                        .html(d.properties.density + " residents per square mile");
                })
                .on('mouseout', function() {
                    tooltip.classed('hidden', true);
                });
;
  	
  svg.append("path")
      .datum(topojson.feature(topology, topology.objects.counties))
      .attr("fill", "none")
      .attr("stroke", "DarkGoldenRod")
      .attr("stroke-opacity", 0.6)
      .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