When using Bar graphs, it can often be the case that there are multiple bars on the same timepoint. This may not always be the desired result. Since groups are plotted one at a time, the allowOverlap option does not work here. For groups you can use the "slots" option.

var options = {
    slots: {
        slot: 1,
        total:3
    }
};

The slots tell the group where to plot it's bar. Each group is given a slot and the total amount of slots. Using this, the bars are plotted side by side neatly.