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;
};

Your progress is currently evenly weighted across all files. You might want to weight the progress by the size of the file, so that if you’re downloading files of vastly different sizes the progress estimation is more accurate. One way you could do that would be to compute the sum loaded and sum total across all tasks, and then use this sum to compute the overall progress—rather averaging per-request progress.

Good catch, Mike. Thanks!