Adaption of Jfire.io animation, for teaching purposes

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
  <style>

</style>
</head>

// created by https://jfire.io/animations/2015-03-09/, adapted for demo
var width = 500,
    height = 500,
    n = 20;
    
var margin = {top: 20, right: 10, bottom: 20, left: 10};
var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height)

var g = svg.selectAll(".groups")
    .data(d3.range(n))
    .enter().append("g").attr("class", "group");

g.append("path")
    .attr("fill", "none")
    .attr("stroke", "#d1d1d1")
    .attr("stroke-width", 3)
    .attr("d", "M-150,0L150,0");

g.selectAll("ellipse")
    .data([-150, 150])
    .enter().append("ellipse")
    .attr("cx", function(d) { return d; })
    .attr("cy", 0)
    .attr("rx", 10)
    .attr("ry", 7)
    .attr("fill", "#bbb");

d3.timer(function(t) {
    g.attr("transform", function(d) {return "translate(" + [width / 2, (d + 1) * height / (n + 1)] + ")scale(" + (Math.sin(d / 2 - t / 1000) + 1) / 2 + ",1)";})
    });

</script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js