Difference Chart

This variation of a bivariate area chart uses clipping to alternate colors. When New York is warmer than San Francisco, the difference between the two is filled in green. When San Francisco is warmer, the difference is filled in red. A similar technique was used by William Playfair all the way back in 1786.

forked from mbostock's block: Difference Chart

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

body {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.x.axis path {
  display: none;
}

.area.above {
  fill: rgb(252,141,89);
}

.area.below {
  fill: rgb(145,207,96);
}

.line {
  fill: none;
  stroke: #000;
  stroke-width: 1.5px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var parseDate = d3.time.format("%Y%m%d").parse;

var x = d3.time.scale()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

var line = d3.svg.area()
    .interpolate("basis")
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d["New York"]); });

var area = d3.svg.area()
    .interpolate("basis")
    .x(function(d) { return x(d.date); })
    .y1(function(d) { return y(d["New York"]); });

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

d3.tsv("data.tsv", function(error, data) {
  if (error) throw error;

data.forEach(function(d) {
    d.date = parseDate(d.date);
    d["New York"]= +d["New York"];
    d["San Francisco"] = +d["San Francisco"];
  });

x.domain(d3.extent(data, function(d) { return d.date; }));

y.domain([
    d3.min(data, function(d) { return Math.min(d["New York"], d["San Francisco"]); }),
    d3.max(data, function(d) { return Math.max(d["New York"], d["San Francisco"]); })
  ]);

svg.datum(data);

svg.append("clipPath")
      .attr("id", "clip-below")
    .append("path")
      .attr("d", area.y0(height));

svg.append("clipPath")
      .attr("id", "clip-above")
    .append("path")
      .attr("d", area.y0(0));

svg.append("path")
      .attr("class", "area above")
      .attr("clip-path", "url(#clip-above)")
      .attr("d", area.y0(function(d) { return y(d["San Francisco"]); }));

svg.append("path")
      .attr("class", "area below")
      .attr("clip-path", "url(#clip-below)")
      .attr("d", area);

svg.append("path")
      .attr("class", "line")
      .attr("d", line);

svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Temperature (ºF)");
});

</script>

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