D3 Examples - circles exercise

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<!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;
   }
    text {font-family:Avenir;font-size:20px}
    rect {fill:green;}
​
  </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)
    
    svg.append("rect")
      .attr("width", 960)
      .attr("height", 500) 
    
    var myData = [1,4,25,67]
    
    svg.append("g").selectAll("circle")
    .data(myData)
    .enter()
    .append("circle")
    .attr("cx",30)
    .attr("cy",50)
    .attr("r",function(d,i){
          return d;
          })
    .attr("opacity",0.2)
    
  </script>
</body>
​
    