Area interpolations

Click to add new points. Hit the DELETE key to remove the selected point. Use the dropdown menu to change the interpolation mode.

<!DOCTYPE html>
<meta charset="utf-8">
<title>Spline Editor</title>
<style>

body {
  font: 13px sans-serif;
  position: relative;
  width: 960px;
  height: 500px;
}

form {
  position: absolute;
  bottom: 10px;
  left: 10px;
}

rect {
  fill: none;
  pointer-events: all;
}

circle,
.line {
  fill: steelblue;
  stroke: steelblue;
  stroke-width: 1.5px;
}

.line {
  stroke-width: 0;
}

circle {
  fill: #fff;
  fill-opacity: .2;
  cursor: move;
}

.selected {
  fill: #ff7f0e;
  stroke: #ff7f0e;
}

</style>
<form>
  <label for="interpolate">Interpolate:</label>
  <select id="interpolate"></select><br>
</form>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var width = 960,
    height = 500;

var points = d3.range(1, 5).map(function(i) {
  return [i * width / 5, 50 + Math.random() * (height - 100)];
});

var dragged = null,
    selected = points[0];

var line = d3.svg.area().y0(height).interpolate('step-after');

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
    .attr("tabindex", 1);

svg.append("rect")
    .attr("width", width)
    .attr("height", height)
    .on("mousedown", mousedown);

svg.append("path")
    .datum(points)
    .attr("class", "line")
    .call(redraw);

d3.select(window)
    .on("mousemove", mousemove)
    .on("mouseup", mouseup)
    .on("keydown", keydown);

d3.select("#interpolate")
    .on("change", change)
   .selectAll("option")
    .data([
      "step-after",
      "step-before",
      "linear",
      "basis",
      "basis-open",
      "basis-closed",
      "cardinal",
      "cardinal-open",
      "cardinal-closed",
      "monotone"
    ])
  .enter().append("option")
    .attr("value", function(d) { return d; })
    .text(function(d) { return d; });

svg.node().focus();

function redraw() {
  svg.select("path").attr("d", line);

var circle = svg.selectAll("circle")
      .data(points, function(d) { return d; });

circle.enter().append("circle")
      .attr("r", 1e-6)
      .on("mousedown", function(d) { selected = dragged = d; redraw(); })
    .transition()
      .duration(750)
      .ease("elastic")
      .attr("r", 6.5);

circle
      .classed("selected", function(d) { return d === selected; })
      .attr("cx", function(d) { return d[0]; })
      .attr("cy", function(d) { return d[1]; });

circle.exit().remove();

if (d3.event) {
    d3.event.preventDefault();
    d3.event.stopPropagation();
  }
}

function change() {
  line.interpolate(this.value);
  redraw();
}

function mousedown() {
  points.push(selected = dragged = d3.mouse(svg.node()));
  redraw();
}

function mousemove() {
  if (!dragged) return;
  var m = d3.mouse(svg.node());
  dragged[0] = Math.max(0, Math.min(width, m[0]));
  dragged[1] = Math.max(0, Math.min(height, m[1]));
  redraw();
}

function mouseup() {
  if (!dragged) return;
  mousemove();
  dragged = null;
}

</script>

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