source: Dev/trunk/d3/src/layout/cluster.js @ 76

// Implements a hierarchical layout using the cluster (or dendogram) algorithm.
d3.layout.cluster = function() {
  var hierarchy = d3.layout.hierarchy().sort(null).value(null),
      separation = d3_layout_treeSeparation,
      size = [1, 1]; // width, height

  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) {
      if (node.children) {
        node.x = d3_layout_clusterX(node.children);
        node.y = d3_layout_clusterY(node.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 - node.y / root.y) * size[1];
    });

    return nodes;
  }

  cluster.separation = function(x) {
    if (!arguments.length) return separation;
    separation = x;
    return cluster;
  };

  cluster.size = function(x) {
    if (!arguments.length) return size;
    size = x;
    return cluster;
  };

  return d3_layout_hierarchyRebind(cluster, hierarchy);
};

function d3_layout_clusterY(children) {
  return 1 + d3.max(children, function(child) {
    return child.y;
  });
}

function d3_layout_clusterX(children) {
  return children.reduce(function(x, child) {
    return x + child.x;
  }, 0) / children.length;
}

function d3_layout_clusterLeft(node) {
  var children = node.children;
  return children ? d3_layout_clusterLeft(children[0]) : node;
}

function d3_layout_clusterRight(node) {
  var children = node.children;
  return children ? d3_layout_clusterRight(children[children.length - 1]) : node;
}