$.ui.plugin.add("draggable", "stack", {
    start: function(event, ui) {
        var group = $.makeArray($(ui.options.stack.group)).sort(function(a,b) {
            return (parseInt($(a).css("zIndex"),10) || ui.options.stack.min) - (parseInt($(b).css("zIndex"),10) || ui.options.stack.min);
        });