SVG Swarm by mbostock

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>SVG Swarm</title>
<style>

svg {
  position: absolute;
  top: 0;
}

circle {
  fill: steelblue;
  stroke: #666;
  stroke-width: 1.5px;
}

</style>
<div id="fps">FPS: <span>?</span></div>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var data = d3.range(500).map(function() {
  return {xloc: 0, yloc: 0, xvel: 0, yvel: 0};
});

var width = 960,
    height = 500;

var x = d3.scale.linear()
    .domain([-5, 5])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([-5, 5])
    .range([0, height]);

var time0 = Date.now(),
    time1;

var fps = d3.select("#fps span");

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

var circle = svg.selectAll("circle")
    .data(data)
  .enter().append("circle")
    .attr("cx", 10)
    .attr("cy", 10)
    .attr("r", 1);

d3.timer(function() {

  data.forEach(function(d) {
    d.xloc += d.xvel;
    d.yloc += d.yvel;
    d.xvel += 0.04 * (Math.random() - .5) - 0.05 * d.xvel - 0.0005 * d.xloc;
    d.yvel += 0.04 * (Math.random() - .5) - 0.05 * d.yvel - 0.0005 * d.yloc;
  });

  circle
      .attr("transform", function(d) { return "translate(" + x(d.xloc) + "," + y(d.yloc) + ")"; })
      .attr("r", function(d) { return Math.min(1 + 1000 * Math.abs(d.xvel * d.yvel), 10); });

  time1 = Date.now();
  fps.text(Math.round(1000 / (time1 - time0)));
  time0 = time1;
});

</script>