Rainbow spiral II

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

path {
  fill: url(#rainbow);
  stroke: #444;
  stroke-width: 1px;
}

</style>
<body>
<script src="//d3js.org/d3.v4.0.0-alpha.18.min.js"></script>
<script>
  var rainbow = d3.scaleRainbow(); // lazy
</script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.14/d3.min.js"></script>
<script>

var width = 960,
    height = 500,
    cx = width / 2,
    cy = height / 2;

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

var gradient = svg.append("defs")
  .append("radialGradient")
    .attr("id","rainbow");

gradient
    .selectAll("stop")
    .data(d3.range(0,1,0.04))
    .enter()
    .append("stop")
      .attr("offset",function(d){
        return d;
      })
      .attr("stop-color",rainbow)
      .each(rotateColor);

var spiral = svg.append("g")
  .each(rotate);

spiral.append("path")
  .attr("d",getSpiral());

function getSpiral() {

var inward = [],
      outward = [],
      rotation = 80 * Math.PI / 180; // rotation of each side from the center spiral

var maxAngle = Math.PI * 2 * 18; // max turn (radians)

var distance = d3.scale.sqrt()
    .domain([0,maxAngle])
    .range([0.25,5]); // distance between turnings

d3.range(0,maxAngle + rotation * 2,0.05).forEach(function(angle){

var a = distance(angle);

var point = [
      cx + (a * angle) * Math.cos(angle),
      cy + (a * angle) * Math.sin(angle)
    ];

inward.unshift(rotatePoint(point,-rotation));
    if (angle < maxAngle) {
      outward.push(rotatePoint(point,rotation));
    }

});

return "M" + inward.join("L") + "L" + outward.join("L") + "Z";

}

function rotatePoint(point,rad) {

var x1 = point[0] - cx,
      y1 = point[1] - cy;

var x2 = x1 * Math.cos(rad) - y1 * Math.sin(rad);
  var y2 = x1 * Math.sin(rad) + y1 * Math.cos(rad);

return [x2 + cx,y2 + cy];

}

function rotateColor() {

d3.select(this).transition()
    .duration(4000)
    .ease("linear")
    .attrTween("stop-color",function(d){
        return function(t) {
          return rainbow((d + t) % 1);
        };
    })
    .each("end",rotateColor);

}

function rotate() {

d3.select(this).transition()
    .duration(4000)
    .ease("linear")
    .attrTween("transform",function(){
        return function(t) {
          return "rotate(" + (t * 360) + " " + cx + " " + cy + ")";
        };
    })
    .each("end",rotate);

}

</script>

https://d3js.org/d3.v4.0.0-alpha.18.min.js

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.14/d3.min.js