Dice roll probabilities

<!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%; }
      text { fill: white; font-family: avenir, sans-serif; text-anchor: middle}
    </style>
  </head>

<body>
    <script>
      
      var SIDES = 6;
      
      function rollDie(){
        return Math.floor(Math.random()*SIDES) + 1;
      }
      
      function indexToPos(i){
        return [Math.floor(i/SIDES) + 1, i % SIDES + 1];
      }
      
      function posToIndex(i,j){
        return (i-1)*SIDES + (j-1);
      }
      
      var margin = {top: 20, right: 10, bottom: 20, left: 10};
      var width = 960 - margin.left - margin.right;
      var 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 + ")");

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