D3 Examples - Area between two curves gradient

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<!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{
  fill: url(#spread-gradient);
}
​
.line {
  fill: none;
  stroke: #C0C0C0;
  stroke-width: 1px;
}
​
​
</style>
<body>
  <div id="legend1"></div>
<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("%d %b %Y").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["VALUE2"]); });
​
var lineVALUE1 = d3.svg.area()
.interpolate("basis")
        .x(function(d) { return x(d.date); })
        .y(function(d) { return y(d["VALUE1"]); });
​
var area = d3.svg.area()
    .interpolate("basis")
    .x(function(d) { return x(d.date); })
    .y1(function(d) { return y(d["VALUE2"]); });
​
​
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.csv("data.csv", function(error, data) {
  if (error) throw error;
var offsets =[];
​
   var quantize = d3.scale.quantize()
    .domain(d3.extent(data, function(d) { return (d["VALUE1"]-d["VALUE2"]); }))
    .range(["#d73027","#f46d43","#fdae61","#fee090","#abd9e9", "#74add1","#4575b4", "#313695"     ]);
​
​
​
   var quantized = d3.scale.linear()
    .domain(d3.extent(data, function(d) { return parseDate(d.date); }))
    .range([0,100]);
​
​
  data.forEach(function(d) {
    d.date = parseDate(d.date);
​
    d["VALUE1"]= +d["VALUE1"];
    d["VALUE2"] = +d["VALUE2"];
​
    offsets.push({offset: Math.round(quantized(d.date),2)+"%", color: quantize(+d["VALUE1"]-d["VALUE2"])})
​
  });
​
  x.domain(d3.extent(data, function(d) { return d.date; }));
​
  y.domain([
    d3.min(data, function(d) { return Math.min(d["VALUE1"], d["VALUE2"])-1; }),
    d3.max(data, function(d) { return Math.max(d["VALUE1"], d["VALUE2"])+1; })
  ]);
​
​
​
  svg.datum(data);
​
  svg.append("path")
      .attr("class", "area")
      .attr("clip-path", "url(#clip-above)")
      .attr("d", area.y0(function(d) { return y(d["VALUE1"]); }));
​
  svg.append("path")
      .attr("class", "line")
      .attr("d", line);
      svg.append("path")
          .attr("class", "line")
          .attr("d", lineVALUE1);
​
  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("Spread");
       x = d3.time.scale()
          .range([0, width]);
​
       y = d3.scale.linear()
          .range([height, 0]);
​
​
    svg.append("linearGradient")
        .attr("id", "spread-gradient")
        .attr("gradientUnits", "userSpaceOnUse")
        .attr("x1", x(0)).attr("y1", y(0))
        .attr("x2", x(1)).attr("y2", y(0))
      .selectAll("stop")
        .data(offsets)
      .enter().append("stop")
        .attr("offset", function(d) { return d.offset; })
        .attr("stop-color", function(d) { return d.color; });
​
​
});
​
​
</script>
​
    