Underscore.js groupBy multiple values

Using Underscore.js, I'm trying to group a list of items multiple times, ie

Group by SIZE then for each SIZE, group by CATEGORY...

http://jsfiddle.net/rickysullivan/WTtXP/1/

Ideally, I'd like to have a function or extend _.groupBy() so that you can throw an array at it with the paramaters to group by.

var multiGroup = ['size', 'category'];

Probably could just make a mixin...

_.mixin({
    groupByMulti: function(obj, val, arr) {
        var result = {};
        var iterator = typeof val == 'function' ? val : function(obj) {
                return obj[val];
            };
        _.each(arr, function(arrvalue, arrIndex) {
            _.each(obj, function(value, objIndex) {
                var key = iterator(value, objIndex);
                var arrresults = obj[objIndex][arrvalue];
                if (_.has(value, arrvalue))
                    (result[arrIndex] || (result[arrIndex] = [])).push(value);

My head hurts, but I think some more pushing needs to go here...

            });
        })
        return result;
    }
});

properties = _.groupByMulti(properties, function(item) {

    var testVal = item["size"];

    if (parseFloat(testVal)) {
        testVal = parseFloat(item["size"])
    }

    return testVal

}, multiGroup);