d3.dispatch = function() {

  var dispatch = new d3_dispatch(),

      i = -1,

      n = arguments.length;

  while (++i < n) dispatch[arguments[i]] = d3_dispatch_event();

  return dispatch;

};

function d3_dispatch() {}

d3_dispatch.prototype.on = function(type, listener) {

  var i = type.indexOf("."),

      name = "";

  // Extract optional namespace, e.g., "click.foo"

  if (i > 0) {

    name = type.substring(i + 1);

    type = type.substring(0, i);

  }

  this[type].on(name, listener);

};

function d3_dispatch_event() {

  var listeners = [],

      listenerByName = {};

  function dispatch() {

    var z = listeners, // defensive reference

        i = -1,

        n = z.length,

        l;

    while (++i < n) if ((l = z[i])._on) l.apply(this, arguments);

  }

  dispatch.on = function(name, listener) {

    var l, i;

    // remove the old listener, if any

    if (l = listenerByName[name]) {

      l._on = false;

      listeners = listeners.slice(0, i = listeners.indexOf(l)).concat(listeners.slice(i + 1));

      delete listenerByName[name];

    }

    // add the new listener, if any

    if (listener) {

      listener._on = true;

      listeners.push(listener);

      listenerByName[name] = listener;

    }

    return dispatch;

  };

  return dispatch;

};