How D3 Transitions Work, Pt. 2: d3.interpolate with multiple elements

Input a set of comma-delimited numbers, and press play to animate.

My quest to understand how transitions work under the hood, part 2: animate multiple elements with different ending x-positions using d3.interpolate and window.requestAnimationFrame.

Part 1 here: How D3 Transitions Work, Pt. 1: d3.interpolate

<html>
  <head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.js" charset="utf-8"></script>
    <style>
    * {
      font-family: Helvetica, Verdana, sans-serif;
    }
    </style>
  </head>

<body>
    <p>
      <input type="text" onkeyup="input(event)" placeholder="100, 200, 300, 400" />
      <button onclick="play()">play</button>
    </p>
    <p>
    <strong>starting x-position</strong> 100 | <strong>ending x-position</strong> <span class="result">[100, 200, 300, 400]</span>
    </p>
    <svg width="1000" height="1000"></svg>

var svg = d3.select('svg');
      var circle;

var start = null;
      var duration = 1000;
      var interpolaters = [];

update();

// enter and exit circles based on the new set of data
      function update() {
        circle = svg.selectAll('circle')
          .data(data);

circle.enter().append('circle');
        circle.exit().remove('circle');

circle.attr('cx', startX)
          .attr('cy', function(d, i) {
            return (i + 1) * yPadding;
          }).attr('r', 25)
          .attr('fill', '#3FB8AF');
      }

// every time user inputs new data
      // update data array and then update circles
      function input(e) {
        data = e.target.value.split(',').map(function(value) {
          return parseFloat(value) || 0;
        });
        d3.select('.result').text(JSON.stringify(data));
        update();
      }

// when user clicks play, reset the circles' x positions
      // create interpolaters based on the data, and animate
      function play() {
        start = null;
        circle.attr('cx', startX);

// create interpolaters for all the elements
        interpolaters = data.map(function(endX) {
          return d3.interpolate(startX, endX);
        });
        window.requestAnimationFrame(step);
      }

// adapted from MDN example for requestAnimationFrame
      // (https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame)
      function step(timestamp) {
        if (!start) start = timestamp;
        var progress = timestamp - start;
        // use calculated interpolaters to calculate cx at time t
        // for each of the circles
        var t = progress / duration;
        circle.attr('cx', function(d, i) {
          return interpolaters[i](t);
        });

if (progress < duration) {
          window.requestAnimationFrame(step);
        }
      }
    </script>
  </body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.js