* transfer commit
* extra_kwargs gets populated
* make extra_kwargs a series kw.
* turn extra_kwargs into Dict of Dicts
* pass to pgfplotsx-backend
* add test
* change to dict
* it even works
* cleanup
* undo pgfplots changes
* all of them
* fix tests
* add pgfplotsx tests and special handling of :add
Co-authored-by: Simon Christ <Simon.Christ@mpikg.mpg.de>
