accept a dictionary in plot colors
- [ ] closes #8193
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Line/bar plot accepts color by dictonary.
@elisamalzoni
Hello @Leostayner! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-19 08:16:24 UTC can you merge master @jreback, Merged! Updated whatsnew. Now where should I update the `.plot` docstring? Here is where I think it should be, is it right?

https://github.com/pandas-dev/pandas/blob/c0050e071d0cc31501d4f730c27a81f3a126fc5c/pandas/plotting/_core.py#L855

https://github.com/pandas-dev/pandas/blob/c0050e071d0cc31501d4f730c27a81f3a126fc5c/pandas/plotting/_core.py#L924 can you merge master Hi @Leostayner - sorry to chase you up, just wanted to ask whether you're still working on this PR :) @MarcoGorelli - given the silence, I think you're okay to finish this up if you want. Sure. Looks like it's gone stale, so I've taken it forward in #31071 (note to OP: you're welcome to reopen this if you want to keep working on it)