D3 Examples - Understanding nest

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<!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 body = d3.select("body");
    var t1 = [[2,45,6,5],[2314,234,234,243],[234,2343242,24323,62],[2321,31,4]];
    var t2 = [[112,5,324,5],[2234,314,234,234,243],[234,2343242,24323,62],[2321,31,4],[325,5235]];
    var t3 = [[12,4335,6,5],[231342,4,22234,234,243],[234,23423,62],[2321,31,234,44]];
    var data =  [t1,t2,t3 ]
    body.selectAll("table")
    .data(data)
    .enter()
    .append("table")
    .selectAll("tr")
    .data(function(d) { return d; })
    .enter()
    .append("tr")
    .selectAll("td")
    .data(function(d) { return d; })
    .enter()
    .append("td")
    .html(function(d){return d;})
  </script>
</body>
​
    