Threshold Key

This example demonstrates how to construct a key from a threshold scale, in the style of Ford Fessenden’s map of police stops involving force. A linear scale is used to set the x-position of each colored rectangle in the key. There is one rectangle per color in the threshold scale’s range, and one tick per value in the threshold scale’s domain. The linear scale’s domain sets the implied extent of the key, here spanning 0 to 100%.

<!DOCTYPE html>
<svg width="960" height="500"><g transform="translate(360,250)"></g></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

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

var threshold = d3.scaleThreshold()
    .domain([0.11, 0.22, 0.33, 0.50])
    .range(["#6e7c5a", "#a0b28f", "#d8b8b3", "#b45554", "#760000"]);

var x = d3.scaleLinear()
    .domain([0, 1])
    .range([0, 240]);

var xAxis = d3.axisBottom(x)
    .tickSize(13)
    .tickValues(threshold.domain())
    .tickFormat(function(d) { return d === 0.5 ? formatPercent(d) : formatNumber(100 * d); });

var g = d3.select("g").call(xAxis);

g.select(".domain")
    .remove();

g.selectAll("rect")
  .data(threshold.range().map(function(color) {
    var d = threshold.invertExtent(color);
    if (d[0] == null) d[0] = x.domain()[0];
    if (d[1] == null) d[1] = x.domain()[1];
    return d;
  }))
  .enter().insert("rect", ".tick")
    .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 threshold(d[0]); });

g.append("text")
    .attr("fill", "#000")
    .attr("font-weight", "bold")
    .attr("text-anchor", "start")
    .attr("y", -6)
    .text("Percentage of stops that involved force");

</script>

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