queue + d3 = asynchronous, parallelized, event-dispatching load queue

load-queue.js

var loadQueue = function() {
  var q = queue(),
      defer = q.defer,
      dispatch = d3.dispatch("progress", "complete"),
      requests = [];

  q.defer = function(load, url) {
    return defer(function(callback) {
      var req = load(url, function(error, data) {
        req.loaded = req.total;
        req.progress = 1;
        update();
        callback.apply(null, arguments);
      })
      .on("progress", function() {
        var e = d3.event;
        req.loaded = e.loaded;
        req.total = e.total;
        req.progress = e.loaded / e.total;
      });
      req.total = req.loaded = req.progress = 0;
      requests.push(req);
    });
  };

  function update() {
    var total = 0,
        loaded = 0,
        progress = 0;
    requests.forEach(function(req) {
      total += req.total;
      loaded += req.loaded;
      progress += req.progress;
    });
    progress /= requests.length;
    dispatch.progress({
      total: total,
      loaded: loaded,
      progress: progress
    });
    if (progress >= 1) {
      dispatch.complete({
        loaded: loaded
      });
    }
  }

  d3.rebind(q, dispatch, "on");
  return q;
};

Good catch, Mike. Thanks!