d3.js bullet chart

<!DOCTYPE html>
<meta charset="utf-8">
<style>
body {
font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
margin: auto;
padding-top: 40px;
position: relative;
width: 800px;
}
button {
position: absolute;
right: 40px;
top: 10px;
}
.bullet { font: 10px sans-serif; }
.bullet .marker { stroke: #000; stroke-width: 2px; }
.bullet .tick line { stroke: #666; stroke-width: .5px; }
.bullet .range.s0 { fill: #eee; }
.bullet .range.s1 { fill: #ddd; }
.bullet .range.s2 { fill: #ccc; }
.bullet .measure.s0 { fill: steelblue; }
.bullet .title { font-size: 14px; font-weight: bold; }
.bullet .subtitle { fill: #999; }
</style>
<button>Update</button>
<script type="text/javascript" src="d3/d3.v3.js"></script>
<script src="js/bullet.js"></script>
<script>
var margin = {top: 5, right: 40, bottom: 20, left: 120},
width = 800 - margin.left - margin.right,
height = 50 - margin.top - margin.bottom;
var chart = d3.bullet()
.width(width)
.height(height);
d3.json("data/cpu1.json", function(error, data) {
var svg = d3.select("body").selectAll("svg")
.data(data)
.enter().append("svg")
.attr("class", "bullet")
.attr("width", width + margin.left + margin.right)
.attr("height", height + margin.top + margin.bottom)
.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")")
.call(chart);
var title = svg.append("g")
.style("text-anchor", "end")
.attr("transform", "translate(-6," + height / 2 + ")");
title.append("text")
.attr("class", "title")
.text(function(d) { return d.title; });
title.append("text")
.attr("class", "subtitle")
.attr("dy", "1em")
.text(function(d) { return d.subtitle; });
d3.selectAll("button").on("click", function() {
svg.datum(randomize).call(chart.duration(1000));
});
});
function randomize(d) {
if (!d.randomizer) d.randomizer = randomizer(d);
d.markers = d.markers.map(d.randomizer);
d.measures = d.measures.map(d.randomizer);
return d;
}
function randomizer(d) {
  var k = d3.max(d.ranges) * .2;
  return function(d) {
    return Math.max(0, d + k * (Math.random() - .5));
    };
  }
</script>
</body>