ColorBrewer Spline

Using uniform one-dimensional b-splines to interpolate in RGB color space, we can convert a discrete color scheme to a continuous one. This shows the “PiYG” scheme from ColorBrewer which has eleven original colors. This scale, and many more, are available in d3-scale-chromatic.

<!DOCTYPE html>
<meta charset="utf-8">
<canvas width="880" height="1" style="width:880px;height:80px;margin:20px 40px;background:#ccc;"></canvas>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg"),
    margin = {top: 40, left: 40, bottom: 40, right: 40},
    width = +svg.attr("width") - margin.left - margin.right,
    height = +svg.attr("height") - margin.top - margin.bottom;

var canvas = d3.select("canvas").node(),
    context = canvas.getContext("2d"),
    canvasWidth = canvas.width;

var x0 = d3.scaleQuantize()
    .domain([0, 1])
    .range(["#8e0152", "#c51b7d", "#de77ae", "#f1b6da", "#fde0ef", "#f7f7f7", "#e6f5d0", "#b8e186", "#7fbc41", "#4d9221", "#276419"]); // PiYG

var x1 = d3.scalePoint()
    .domain(x0.range())
    .range([0, width]);

var x2 = d3.scaleLinear()
    .domain([0, 1])
    .range([0, width]);

var y = d3.scaleLinear()
    .domain([0, 255])
    .range([height, 0]);

var g = svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

g.append("g")
  .selectAll("circle")
    .data(x1.domain())
  .enter().append("circle")
    .attr("cx", function(d) { return x1(d); })
    .attr("r", 20)
    .attr("fill", function(d) { return d; });

g.append("g")
    .attr("class", "axis axis--x")
    .call(d3.axisTop(x1).tickPadding(18));

g.append("g")
    .attr("class", "axis axis--y")
    .call(d3.axisLeft(y));

g.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x2));

line.append("path")
    .attr("stroke-width", 3)
    .attr("d", function(values) {
      var i = d3.interpolateBasis(values.map(function(v) { return v[1]; }));
      return d3.line()
          .x(function(t) { return x2(t); })
          .y(function(t) { return y(i(t)); })
          (d3.range(0, 1 + 1e-6, 0.001));
    });

line.append("path")
    .attr("stroke-dasharray", "2,2")
    .attr("stroke", "#000")
    .attr("d", d3.line()
        .curve(d3.curveLinear)
        .x(function(d) { return x2(d[0]); })
        .y(function(d) { return y(d[1]); }));

var image = context.createImageData(canvasWidth, 1),
    interpolate = d3.interpolateRgbBasis(x0.range());

for (var i = 0, k = 0; i < canvasWidth; ++i, k += 4) {
  var c = d3.rgb(interpolate(i / (canvasWidth - 1)));
  image.data[k] = c.r;
  image.data[k + 1] = c.g;
  image.data[k + 2] = c.b;
  image.data[k + 3] = 255;
}

context.putImageData(image, 0, 0);

</script>

https://d3js.org/d3.v4.min.js