Use this function to turn on data-stacking. If $stackWhat is supplied only specific data-types will be stacked. Please note that possibly not all data-types might support stacking, in which case they will simply be left unstacked. But the base-types "bar" and "line" do support it! If you use both Y-axes then only data on the same axis can be stacked (sure!). So if you have e.g. line-data on both axes they will be stacked independently.