US Population by Metro Area.

You can recreate the data by running make in the cloned directory.

The key files are: MetroArea.csv us.json and ACS_12_5YR_B01003_with_ann.csv

See the Makefile for how they were created or downloaded.

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

.land {
fill: #ddd;
}

.border {
fill: none;
stroke: #fff;
stroke-linejoin: round;
stroke-linecap: round;
}

.bubble {
fill: brown;
fill-opacity: .5;
stroke: #fff;
stroke-width: .5px;
}

.bubble :hover {
stroke: #000;
}

.legend circle {
fill: none;
stroke: #ccc;
}

.legend text {
fill: #777;
font: 10px sans-serif;
text-anchor: middle;
}

</style>
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="https://d3js.org/queue.v1.min.js"></script>
<script src="https://d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 600;

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

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

var radius = d3.scale.sqrt()
    .domain([0, 1e6])
    .range([0, 15]);

var formatNumber = d3.format(",.0f");

var metroAreaByID = d3.map();
var populationByMetroArea = d3.map();

queue(1)
  .defer(d3.csv,"MetroArea.csv", function(d) {
      if(d["Metropolitan Division Title"]) {
        metroAreaByID.set( d["FIPS State Code"] + d["FIPS County Code"],{id: d["Metro Division Code"], name: d["Metropolitan Division Title"]});
      } else {
        metroAreaByID.set( d["FIPS State Code"] + d["FIPS County Code"],{id: d["CBSA Code"], name: d["CBSA Title"]});
      }
  })
  .defer(d3.csv,"ACS_12_5YR_B01003_with_ann.csv", function(d) {
    if(metroAreaByID.has(d["geoid"])) {
      var metroName = metroAreaByID.get(d["geoid"]).name;
      if(populationByMetroArea.has(metroAreaByID.get(d["geoid"]).id)) {
        var current = populationByMetroArea.get(metroAreaByID.get(d["geoid"]).id);
        if (Number(d.population) > current.maxpop){
          populationByMetroArea.set(metroAreaByID.get(d["geoid"]).id,{ name: metroName, population: current.population + Number(d.population), maxpop: Number(d.population), id: d.geoid });
        } else {
          populationByMetroArea.set(metroAreaByID.get(d["geoid"]).id,{ name: current.name, population: current.population + Number(d.population), maxpop: current.population, id: current.id });
        }
      } else {
        populationByMetroArea.set(metroAreaByID.get(d["geoid"]).id,{ name: metroName, population: Number(d.population), maxpop: Number(d.population), id: d.geoid });
      }
    } else {
      populationByMetroArea.set(d.geoid,{ name: d.name, population: Number(d.population), maxpop: Number(d.population), id: d.geoid});
    }
  })
  .defer(d3.json,"us.json", function(error, us) {
    if (error) return console.error(error);

svg.append("path")
        .datum(topojson.feature(us, us.objects.nation))
        .attr("class", "land")
        .attr("d", path);

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

svg.append("g")
      .attr("class", "bubble")
      .selectAll("circle")
      .data(topojson.feature(us, us.objects.counties).features
        .sort(function(a, b) { return b.properties.population - a.properties.population; }))
      .enter().append("circle")
      .attr("transform", function(d) { return "translate(" + path.centroid(d) + ")"; })
      .attr("r", function(d) {
          if(populationByMetroArea.has(d.id)) {
            return radius(populationByMetroArea.get(d.id).population);
          } else {
            if(d.id === populationByMetroArea.get(metroAreaByID.get(d.id).id).id) {
              return radius(populationByMetroArea.get(metroAreaByID.get(d.id).id).population);
            } else {
              return 0;
            }
          }
      })
      .append("title")
      .text(function(d) {
          if(populationByMetroArea.has(d.id)) {
            return populationByMetroArea.get(d.id).name + formatNumber(populationByMetroArea.get(d.id).population);
          } else {
            if(d.id === populationByMetroArea.get(metroAreaByID.get(d.id).id).id) {
              return populationByMetroArea.get(metroAreaByID.get(d.id).id).name + "\nPopulation " + formatNumber(populationByMetroArea.get(metroAreaByID.get(d.id).id).population);
            } else {
              return "";
            }
          }
      }); 
});

var legend = svg.append("g")
    .attr("class", "legend")
    .attr("transform", "translate(" + (width - 50) + "," + (height - 20) + ")")
  .selectAll("g")
    .data([1e6, 3e6, 6e6])
  .enter().append("g");

legend.append("circle")
    .attr("cy", function(d) { return -radius(d); })
    .attr("r", radius);

legend.append("text")
    .attr("y", function(d) { return -2 * radius(d); })
    .attr("dy", "1.3em")
    .text(d3.format(".1s"));

</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

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

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

https://d3js.org/d3.v3.min.js

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

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