D3 Examples - force drop-down histogram

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <script src="d3-forceextent.js"></script>
  <style>
    body { margin: 0; position: fixed; top: 0; right: 0; bottom: 0; left: 0; }
    
    body {
      font-family: monospace;
    }
    
    circle {
      fill: teal;
      stroke-width: 0;
    }
    
    .coin-text {
      opacity: 0.5;
    }
    
    .counter {
      font-size: 20px;
    }
    
    .animate {
      color: black;
      position: absolute;
      top: 10px;
      left: 10px;
    }
  </style>
</head>
​
<body>
  <script>
    var margin = { top: 50, right: 50, bottom: 50, left: 50 };
    
    var width = 960 - margin.left - margin.right,
        height = 500 - margin.top - margin.bottom;
    
    var svg = d3.select("body").append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom)
    .append("g")
        .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
    
    var noOfCirclesA = 10;
        noOfCirclesB =100;
    
    var aPos = width / 7;
            bPos = width - width / 4;
    
    var te = d3.easeBounce;
    
    var radius = 15;
    
    var dataA = d3.range(noOfCirclesA);
    dataA = dataA.map(function(d) {
      return {
        id: d
      }
    });
    
    var dataB = d3.range(noOfCirclesB);
    dataB = dataB.map(function(d) {
      return {
        id: d
      }
    });
    
    var extent = [[0, width], [0, height]];
    
    var simulationA = d3.forceSimulation(dataA)
        .force("x", d3.forceX(aPos))
        .force("y", d3.forceY(height))
        .force("extent", d3.forceExtent(extent))
        .force("collide", d3.forceCollide(radius + 2))
        .stop(); 
    
    var simulationB = d3.forceSimulation(dataB)
        .force("x", d3.forceX(bPos))
        .force("y", d3.forceY(height))
        .force("extent", d3.forceExtent(extent))
        .force("collide", d3.forceCollide(radius + 2))
        .stop();  
    
    for (var i = 0; i < 120; ++i) simulationA.tick();
    for (var i = 0; i < 120; ++i) simulationB.tick();
    
    var counterA = 0,
        counterB = 0;
    
    var legendA = svg.append("g")
        .append("text")
        .attr("class", "counter")
        .attr("x", aPos)
        .attr("text-anchor", "middle")
        .text(0);
    
    var legendB = svg.append("g")
        .append("text")
        .attr("class", "counter")
        .attr("x", bPos)
        .attr("text-anchor", "middle")
        .text(0);
    
    function increment(pile) {
      if (pile === "a") {
        counterA++;
        legendA.text(counterA);
      } else {
        counterB++;
        legendB.text(counterB);
      }
    }
    
    addCircles(dataA, "a");
    addCircles(dataB, "b");
    
    function addCircles(data, pile) {
      
      var circle = svg.append("g")
        .attr("class", "circles")
        .selectAll("g").data(data)
      .enter().append("g")
        .attr("transform", function(d) {
          return "translate(" + d.x + "," + (-margin.top - radius * 2) + ")"
        });
    
      circle.append("circle")
        .attr("r", radius) 
      circle.append("text")
        .attr("class", "coin-text")
        .attr("text-anchor", "middle")
        .attr("dy", radius / 3)
        .text("£");
​
      circle.transition()
        .duration(750)
        .delay(function(d, i) {
          return (height - d.y) * 20;
        })
//         .ease(te)
        .attr("transform", function(d) {
          return "translate(" + d.x + ", " +  d.y + ")";
        })
        .on("end", function() {
          increment(pile);
        });
    }
        
  </script>
</body>
​
    