<!DOCTYPE html>
<meta charset="utf-8">
<style>
circle {
fill: none;
stroke: steelblue;
}
</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>
var width = 960,
height = 500;
var path = d3.geo.path();
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
d3.json("/mbostock/raw/4090846/us.json", function(error, us) {
if (error) throw error;
svg.selectAll("circle")
.data(topojson.feature(us, us.objects.counties).features)
.enter().append("circle")
.attr("transform", function(d) { return "translate(" + path.centroid(d) + ")"; })
.attr("r", function(d) { return Math.sqrt(path.area(d) / Math.PI); });
});
</script>