since this is what also will be used for ConfusionMatrix plotting I guess,  it might be nice to add option to allow groupping with a separator by empty rows/columns (in ConfusionMatrix now it is achieved by providing None in the list of labels)... not sure yet how it should be interfaced here... may be ```x/ylabel_attrs_space_after``` option providing lists of those boundary elements? although would then need to be also allowing lists of lists so to specify pairs?  just an idea anyways