harmonic motion

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<canvas width="960" height="500"></canvas>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var canvas = document.querySelector("canvas"),
    context = canvas.getContext("2d"),
    width = canvas.width,
    height = canvas.height,
    step = 0.015;

context.translate(width / 2, height / 2);
context.lineWidth = 0.30;
context.strokeStyle = "#000000";

var scales = {
  phase: d3.scaleLinear().range([0, Math.PI]),
  damping: d3.scaleLinear().range([0.015, 0.015]),
  frequency: d3.scaleLinear().range([5, 8])
};

var keys = d3.keys(scales);

var x = new Pendulum(),
    y = new Pendulum(),
    xy = new Pendulum();

x.amplitude = y.amplitude = 125;
xy.amplitude = 125;

tween();

function tween() {
  d3.transition()
    .duration(4000)
    .tween("tween", function(){

var p = choose([x, y, xy]),
          scale = keys.shift(),
          invert = p[scale] === scales[scale](1);

keys.push(scale);

return function(t){
        p[scale] = scales[scale](invert ? 1 - t : t);
        draw();
      };
    })
    .on("end", tween);
}

function draw(){

var t = 0;

context.clearRect(-width / 2, -height / 2, width, height);
  context.beginPath();
  x.stopped = y.stopped = xy.stopped = false;

while (!x.stopped || !y.stopped || !xy.stopped) {
    context[t ? "lineTo" : "moveTo"](x.pos(t) + xy.x(t), y.pos(t) + xy.pos(t));
    t += step;
  }

context.stroke();

}

function Pendulum() {
  for (var key in scales) {
    this[key] = scales[key](Math.random() < 0.75 ? 0 : 1);
  }
}

Pendulum.prototype.pos = function(t){
  return this.amplitude * Math.sin(t * this.frequency + Math.PI * this.phase) * this.damp(t);
};

Pendulum.prototype.damp = function(t){
  var factor = Math.exp(-this.damping * t);
  if (factor < 0.1) {
    this.stopped = true;
  }
  return factor;
};

Pendulum.prototype.x = function(t){
  return this.amplitude * Math.cos(t * this.frequency) * this.damp(t);
};

function choose(arr){
  return arr[Math.floor(Math.random() * arr.length)];
}

</script>

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