Merge Sites, One Move

In answer to a question on the d3 Slack channel #help

See the more complex version, with transitions for each step, at Merge Sites.

<!DOCTYPE html> 
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
    // Feel free to change or delete any of the code you see in this editor!
    var svg = d3.select("body").append("svg")
      .attr("width", 960)
      .attr("height", 500)

var data = d3.range(300).map(d => {
      return {
        x: 100 + 760 * Math.random(),
        y: 100 + 300 * Math.random(),
        w: 300 * Math.random(),
      }
    })

while(merge());

circles.transition()
      .duration(2000)
      .attr('r', d => Math.sqrt(d.w))
      .attr('cx', d => d.x)
      .attr('cy', d => d.y);
      circles.filter(d => !d.w)
        .transition()
        .duration(2000)
        .attr('opacity', 0)
        .remove();
    
    
  </script>
</body>

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