D3 Examples - Bike Peak Data, ways of looking at rates of growth

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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;
}
​
path.line {
    stroke-width:4;
    fill:none;
    stroke:darkblue;
}
​
path.linear {
    stroke-width:2;
    fill:none;
    stroke:gray;
    stroke-dasharray:2,2;
}
​
</style>
<body>
<script src="https://d3js.org/d3.v3.js"></script>
<script>
​
var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 400 - margin.left - margin.right,
    height = 250 - margin.top - margin.bottom;
​
var parseDate = d3.time.format("%Y").parse;
​
var x = d3.time.scale()
    .range([0, width]);
​
var y = d3.scale.linear()
    .range([height, 0]);
​
var x2 = d3.time.scale()
    .range([0, width]);
​
var y2 = d3.scale.linear()
    .range([height, 0]);
​
var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");
​
var x2Axis = d3.svg.axis()
    .scale(x2)
    .orient("bottom");
​
var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");
​
var y2Axis = d3.svg.axis()
    .scale(y2)
    .orient("left");
​
var absolute = 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 + ")");
​
var line = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.peak); });
​
var line2 = d3.svg.line()
    .x(function(d) { return x2(d.date); })
    .y(function(d) { return y2(d.peak); });
​
var added = 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("peak.csv", function(error, data) {
  data.forEach(function(d) {
    d.date = parseDate(d.year);
    d.peak = +d['Peak Hour'];
  });
​
  x.domain(d3.extent(data, function(d) { return d.date; }));
  y.domain(d3.extent(data, function(d) { return d.peak; }));
​
  var linear = absolute.selectAll(".linear")
    .data([[{ date: x.domain()[0], peak: y.domain()[0] }, { date: x.domain()[1], peak: y.domain()[1] }]])
    .enter().append("g")
      .attr("class", "linear");
​
  linear.append("path")
      .attr("class", "linear")
      .attr("d", line);
​
  var l = absolute.selectAll(".browser")
      .data([data])
    .enter().append("g")
      .attr("class", "browser");
​
  l.append("path")
      .attr("class", "line")
      .attr("d", line);
​
  absolute.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);
​
  absolute.append("g")
      .attr("class", "y axis")
      .call(yAxis);
​
  var added_data = [];
  for (var i = 1; i < data.length; i++) {
      added_data.push({
          date: data[i].date,
          peak: data[i].peak - data[i - 1].peak
      });
  }
​
  x2.domain(d3.extent(added_data, function(d) { return d.date; }));
  y2.domain(d3.extent(added_data, function(d) { return d.peak; }));
​
  added.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(x2Axis);
​
  added.append("g")
      .attr("class", "y axis")
      .call(y2Axis);
​
  var ladd = added.selectAll(".foo")
      .data([added_data])
    .enter().append("g")
      .attr("class", "foo");
​
  ladd.append("path")
      .attr("class", "line")
      .attr("d", line2);
​
});
​
</script>
​
    