(function() {

  // get a reference to the d3.selection prototype,
  // and keep a reference to the old d3.selection.on
  var d3_selectionPrototype = d3.selection.prototype,
      d3_on = d3_selectionPrototype.on;

  // our shims are organized by event:
  // "desired-event": ["shimmed-event", wrapperFunction]
  var shims = {
    "mouseenter": ["mouseover", relatedTarget],
    "mouseleave": ["mouseout", relatedTarget]
  };

  // rewrite the d3.selection.on function to shim the events with wrapped
  // callbacks
  d3_selectionPrototype.on = function(evt, callback, useCapture) {
    var bits = evt.split("."),
        type = bits.shift(),
        shim = shims[type];
    if (shim) {
      evt = bits.length ? [shim[0], bits].join(".") : shim[0];
      if (typeof callback === "function") {
        callback = shim[1](callback);
      }
      return d3_on.call(this, evt, callback, useCapture);
    } else {
      return d3_on.apply(this, arguments);
    } 
  };

  function relatedTarget(callback) {
    return function() {
      var related = d3.event.relatedTarget;
      if (this === related || childOf(this, related)) {
        return undefined;
      }
      return callback.apply(this, arguments);
    };
  }

  function childOf(p, c) {
    if (p === c) return false;
    while (c && c !== p) c = c.parentNode;
    return c === p;
  }

})();