Follow the Mouse

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
    svg { width:100%; height: 100% }
  </style>
</head>

<body>
  <script>
    var width = 800, height = 500;
    var stripWidth = 20, stripCount = 0;
    var radius = 30, moveCount = 0, index = 0;
    var factor = 0, factors = [1.0, 0.5];
    
    var circleColors = ["#467864", "#C45F4B", "#86C4AC", "#5FC49C", "#784E46"];
    
    var svg = d3.select("body")
    .append("svg")
    .attr("width", width)
    .attr("height", height);
    
    svg.on("mousemove", function() {
      index = (++moveCount) % circleColors.length;
      stripCount = Math.floor(moveCount/stripWidth);
      factor = factors[stripCount%2];
      
      var circle = svg.append("circle")
      .attr("cx", d3.event.pageX)
      .attr("cy", d3.event.pageY)
      .attr("r", radius*factor);
      
      circle.attr("fill", circleColors[index]);
    })
  </script>
</body>

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