We do not have properly grouped data. So where plots  look right, it has always been by coincidence that the `groupby` required by the matplotlib functions sometimes coincide with the aesthetic mappings. Related, I will be changing `geom._groups` to `geom._units` to remove any misunderstanding.

Real groups will be assembled in `layer` (from the `ggplot.aesthetics` and `geom.aes` or using the `group` parameter), do a `groupby` for the `stats` and the resulting data assembled back into a single dataframe.

To get right plots, a temporary hack (like the many currently in place) will be to steal the ggplot aesthetic mappings in the `__radd__` functions and create usable groups.