d3.random = {
  normal: function(mean, deviation) {
    if (arguments.length < 2) deviation = 1;
    if (arguments.length < 1) mean = 0;
    return function() {
      var x, y, r;
      do {
        x = Math.random() * 2 - 1;
        y = Math.random() * 2 - 1;
        r = x * x + y * y;
      } while (!r || r > 1);
      return mean + deviation * x * Math.sqrt(-2 * Math.log(r) / r);
    };
  }
};