Custom interpolator for path

This example use a custom path interpolator, as opposed to the other examples.

The interpolator interpolates the number of elements of the array and inserts missing elements

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

.frame {
  fill: none;
  stroke: #000;
}

.axis text {
  font: 10px sans-serif;
}

.axis line,
.axis circle {
  fill: none;
  stroke: #777;
  stroke-dasharray: 1,4;
}

.axis :last-of-type circle {
  stroke: #333;
  stroke-dasharray: none;
}

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

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

d3.interpolators.push(function(a,b){
    if (Array.isArray(a[0]) && Array.isArray(b[0])) {
      var x = [],
      c = [],
      na = a.length,
      nb = b.length,
      n0 = Math.min(a.length, b.length),
      i,
      delta = b.length - a.length,
      newSize;
       console.log('No',na,nb);
     
      for (i=0; i < na; ++i)  
      {
        c[i]=b[i];
      }
      newSize;
        return function(t) {
          newSize  = a.length + delta*t;
          //console.log(newSize,c.length,t,delta);
          if (newSize>c.length){
             for (i=c.length;i<newSize;i++) c.push(b[i]);
          }
          return c;
        }
    }
});

function d3_interpolateArray(a, b) {
  //we suppose a is smaller
  var x = [],
      c = [],
      na = a.length,
      nb = b.length,
      n0 = Math.min(a.length, b.length),
      i,
      delta = b.length - a.length,
      newSize;
  for (i = 0; i < n0; ++i) x.push(d3_interpolate([0,0], b[i]));
  for (i=0; i < na; ++i) c[i] = a[i];
  return function(t) {
    newSize  = a.length + delta*t;
    if (newSize>c.length) c.push(b[newSize]);
    for (i = 0; i < c.length; ++i) c[i] = x[i](t);
    return c;
  };
}

var data = d3.range(0, 2*Math.PI, .01).map(function(t) {
  return [t, Math.sin(4*t)];
});
var empty =  d3.range(0, 2*Math.PI, .01).map(function(t) {
  return [0,0];
});

var margin = 30;
// radius is the minimal dimension, minus the margin
var width = 960,
    height = 500,
    radius = Math.min(width, height) / 2 - margin;

var r = d3.scale.linear()
    .domain([0, 1])
    .range([0, radius]);

//default accessor [[x1,y1]] => radian and angle
var line = d3.svg.line.radial()
    .radius(function(d){; return (r(d[1])); })  // will change between -1 and 1
    .angle(function(d) { ;return d[0];});

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)
  .append("g")
  .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");
// radius axis
// cheat with CSS
var gr = svg.append("g")
  .attr("class", "r axis")
  .selectAll("g")
  .data(r.ticks(10).slice(0))
  .enter().append("g");

gr.append("circle")
    .attr("r", function(d,i){console.log("=>",d,i,r(d));return r(d)});

gr.append("text")
    .attr("y", function(d) { return -r(d) - 4; })
    .attr("transform", "rotate(50)")
    .style("text-anchor", "middle")
    .text(function(d) { return d; });

var ga = svg.append("g")
  .attr("class", "a axis")
  .selectAll("g")
    .data(d3.range(0, 360, 15))
  .enter().append("g")
    .attr("transform", function(d) { return "rotate(" + (d-90) + ")"; });

ga.append("line")
    .attr("x2", radius);

ga.append("text")
    .attr("x", radius + 6)
    .attr("dy", ".35em")
    .style("text-anchor", function(d) { return d < 270 && d > 90 ? "end" : null; })
    .attr("transform", function(d) { return d < 270 && d > 90 ? "rotate(180 " + (radius + 6) + ",0)" : null; })
    .text(function(d) { return d + "°"; });
// a bit cheating
var path =
     svg.append("path")
    .datum(data)
    .attr("class", "line")
    .attr("d", line);
var totalLength = path.node().getTotalLength();

path.transition().duration(1000).attrTween("d",function(d){
  console.log(empty,d);
  var interpolate = d3.interpolate( [[0,0]],d);
  return function(t) { return line(interpolate(t))};
})

</script>
</body>

Modified http://d3js.org/d3.v3.min.js to a secure url

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