<script type="text/javascript">
    var width = 960, height = 500;

    var color = d3.scale.threshold() // <-A
            .domain([.02, .04, .06, .08, .10])
            .range(["#f2f0f7", "#dadaeb", "#bcbddc", 
             "#9e9ac8", "#756bb1", "#54278f"]);

    var path = d3.geo.path();

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

var g = svg.append("g")
...
    d3.json("/data/us.json", function (error, us) {
        d3.tsv("/data/unemployment.tsv", function (error, 
                                            unemployment) {
            var rateById = {};
            unemployment.forEach(function (d) { // <-B
                rateById[d.id] = +d.rate;
            });

            g.append("g")
                    .attr("class", "counties")
                    .selectAll("path")
                    .data(topojson.feature(us, 
                            us.objects.counties).features)
                    .enter().append("path")
                    .attr("d", path)
                    .style("fill", function (d) {
                        return color(rateById[d.id]); // <-C
                    });

            g.append("path")
                    .datum(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; }))
                    .attr("class", "states")
                    .attr("d", path);
        });
});
...
</script>