Add lines.markerfacecoloralt/mfcalt to rcParams

[kalagau]

Bug report

Bug summary

lines has an option to set markerfacecoloralt/mfcalt but that option isn't available via rcParams['lines.markerfacecoloralt']
Code for reproduction

import matplotlib as mpl
mpl.rcparams['lines.markerfacecolor']

Actual outcome

 KeyError: 'lines.markerfacecoloralt'

Expected outcome
The default value of markerfacecoloralt should be returned

Matplotlib version

• Operating system:Ubuntu 17.1
• Matplotlib version: 2.1.2
• Matplotlib backend (print(matplotlib.get_backend())): GTK3Agg
• Python version:3.6

[kalagau]

Would it be possible to have this confirmed? I would like to start working on this.

[jklymak]

I don't think we garauntee that every kwarg will get an rcParam. I don't even know what markerfacecoloralt is supposed to refer to, and the documentation is not much help. I'd have to be convinced this needs to be in the rcParam list, perhaps by someone better dcoumenting what it does and why.

[timhoffm]

There are two-color fill styles for markers:
https://matplotlib.org/gallery/lines_bars_and_markers/marker_fillstyle_reference.html

[jklymak]

Cool. I'd never have gotten there from the docstrings for those params, so labelling as a documentation issue. Regardless, I don't know that this needs an rcParam.

[kalagau]

I was under the impression that adding this would improve consistency. Since there are already rcparams for mfc.

[timhoffm]

Probably not particularly important, but I don't see why we should leave this out. So +0.5 for adding this.

By the way, am I correct that styles are based on rcParams? If so, +1 for adding. This should be configurable in a style.

[jklymak]

I guess you could make that argument. The counter would be that you already have to specify the fillstyle, so specifying the color is hardly onerous.

However, I'd take the slightly longer rcParam list if the same PR also explains properly what these do in the docstring of the parameters. Its completely opque unless you look at the example above.

[QuLogic]

I'm rather confused about what needs to be documented here, as it doesn't seem to be what the title is referencing.

[timhoffm]

This was mislabeled. It's a request to add a new rcParam.

[github-actions]

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