D3 Examples - Area Gradient fill

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<!DOCTYPE html>
<meta charset="utf-8">
<style>
​
body {
  margin: auto;
  width: 960px;
}
​
text {
  font: 10px sans-serif;
}
​
.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}
​
.x.axis path {
  display: none;
}
​
.area {
  fill: url(#temperature-gradient);
  stroke-width: 05px;
}
​
</style>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.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  area = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.temperature); });
​
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) {
  data.forEach(function(d) {
    d.date = parseDate(d.date);
    d.temperature = +d.temperature;
  });
​
  x.domain([data[0].date, data[data.length - 1].date]);
  y.domain(d3.extent(data, function(d) { return d.temperature; }));
​
  svg.append("linearGradient")
      .attr("id", "temperature-gradient")
      .attr("gradientUnits", "userSpaceOnUse")
      .attr("x1", 0).attr("y1", y(50))
      .attr("x2", 0).attr("y2", y(60))
    .selectAll("stop")
      .data([
        {offset: "0%", color: "steelblue"},
        {offset: "50%", color: "gray"},
        {offset: "100%", color: "red"}
      ])
    .enter().append("stop")
      .attr("offset", function(d) { return d.offset; })
      .attr("stop-color", function(d) { return d.color; });
​
  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)");
​
    svg.append("path")
        .datum(data)
        .attr("class", "area")
        .attr("d", area);
​
});
​
</script>
​
    