ENH: Allow column grouping in DataFrame.plot
- [X] closes #29688
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This is an attempt at #29688 i.e. group some columns on the same ax when calling `df.plot`

See [this gist](https://gist.github.com/NicolasHug/4d1f60212ba69e6000982951ae47f203) for illustration of different supported plots

Thanks!