Sankey Interpolation

A custom line interpolator with horizontal tangents.

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

body {
  font: 10px sans-serif;
}

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

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

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

var margin = {top: 20, right: 30, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scale.linear()
    .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.line()
    .interpolate(interpolateSankey)
    .x(function(d) { return x(d.x); })
    .y(function(d) { return y(d.y); });

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;

data.forEach(function(d) {
    d.x = +d.x;
    d.y = +d.y;
  });

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

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

svg.append("g")
      .attr("class", "y axis")
      .call(yAxis);

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

function interpolateSankey(points) {
  var x0 = points[0][0], y0 = points[0][1], x1, y1, x2,
      path = [x0, ",", y0],
      i = 0,
      n = points.length;
  while (++i < n) {
    x1 = points[i][0], y1 = points[i][1], x2 = (x0 + x1) / 2;
    path.push("C", x2, ",", y0, " ", x2, ",", y1, " ", x1, ",", y1);
    x0 = x1, y0 = y1;
  }
  return path.join("");
}

</script>

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