Feat/scatter by size
- [y ] partial solution to (only size_by_variable is handled, not color_by_variable) #16827 
- [y] tests added: scatter_test.py shows an example of use
- [y] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [n] whatsnew entry

Let me know if any changes are needed.
Thanks!
Vincent