D3 Examples - Enter and Append Circles

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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; }
    svg {
      width: 100%;
      height: 100%;
    }
  </style>
</head>
​
<body>
  <script>
    var myData = [1,2,3,4,5];
    
    var svgViewport = d3.select("body")
        .append("svg")
        .attr("width","560")
        .attr("height","900");
    
    // Update
    var circleSelection = svgViewport.selectAll("circle").data(myData);
    
    // Enter and Append
    var circleElements = circleSelection.enter().append("circle");
    
    // Set properties for each circle
    circleElements
      .attr("cx",function(d,i) { return d * 50; })
        .attr("cy",function(d,i) { return d * 50; })
        .attr("r","25");
    
    function colorCircles(d,i) {
      return i % 2 === 0 ? 'green' : 'red';
        }
    
    // Style each circle
    var circleStyle = circleElements.style("fill", colorCircles);
  </script>
</body>
​
    