code: function cluster(d, i) {
      var nodes = hierarchy.call(this, d, i),
          root = nodes[0],
          previousNode,
          x = 0,
          kx,
          ky;
  
      // First walk, computing the initial x & y values.
      d3_layout_treeVisitAfter(root, function(node) {
        var children = node.children;
        if (children && children.length) {
          node.x = d3_layout_clusterX(children);
          node.y = d3_layout_clusterY(children);
        } else {
          node.x = previousNode ? x += separation(node, previousNode) : 0;
          node.y = 0;
          previousNode = node;
        }
      });
  
      // Compute the left-most, right-most, and depth-most nodes for extents.
      var left = d3_layout_clusterLeft(root),
          right = d3_layout_clusterRight(root),
          x0 = left.x - separation(left, right) / 2,
          x1 = right.x + separation(right, left) / 2;
  
      // Second walk, normalizing x & y to the desired size.
      d3_layout_treeVisitAfter(root, function(node) {
        node.x = (node.x - x0) / (x1 - x0) * size[0];
        node.y = (1 - (root.y ? node.y / root.y : 1)) * size[1];
      });
  
      return nodes;
    }
docs: width, height