Allow both 'on data' legend together with legend on righ-hand side of plot

What kind of feature would you like to request?

Additional function parameters / changed functionality / changed defaults?

Please describe your wishes

When plotting with "on data" legends, it's not always easy to see which label belongs to which color in the plot. Including the right-hand legend together with the "on data" labels could be a simple fix to improve readability.

I suggest to change the following lines:

https://github.com/scverse/scanpy/blob/391d87ad50d015d1f937d7d993dad36f24b915a5/src/scanpy/plotting/_tools/scatterplots.py#L1141-L1144

as follows:

    if legend_loc not in {None, "none"}:
        for label in cats:
            ax.scatter([], [], c=palette[label], label=label)
        if legend_loc in {"on data", "right margin"}:

Note: I didn't consider any flexibility in preserving the old behaviour and/or extending the parameters to a new legend location descriptor. Personally I feel that would unnecessarily complicate the setting, but I also understand if defaults shouldn't change.

Start in src/scanpy/plotting/_tools/scatterplots.py around lines 1141-1144 and trace how legend_loc controls the plotted legend. The change is complete when using "on data" can also display the legend on the right-hand side without changing the intended behavior of other legend locations.