Adds style related dictionaries to plot_decision_regions

[jrbourbeau]

Description

This PR adds parameters to plot_decision_regions that can be used to customize the plotting style used (e.g. size of scatter plot markers, alpha value of decision regions, etc.)

Related issues or pull requests

Fixes #340

Pull Request requirements

• Added appropriate unit test functions in the ./mlxtend/*/tests directories
• Ran nosetests ./mlxtend -sv and make sure that all unit tests pass
• Checked the test coverage by running nosetests ./mlxtend --with-coverage
• Checked for style issues by running flake8 ./mlxtend
• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file
• Modify documentation in the appropriate location under mlxtend/docs/sources/ (optional)
• Checked that the Travis-CI build passed at https://travis-ci.org/rasbt/mlxtend

[jrbourbeau]

Additional TODO:

• Make user can't override markers and colors parameters in plot_decision_regions with scatter_kwargs, etc.

[coveralls]

Coverage increased (+0.2%) to 91.727% when pulling 3990f22 on jrbourbeau:add_decision_regions_kwargs into 4f148c4 on rasbt:master.

[rasbt]

That's a nice function dealing with the issues of protected keys! One little thing that comes to mind is that the way I wrote the API parser for the documentation, this would show up in the plotting API docs as well. You could make this function private by prefixing it with an underscore (_format_plotting_kwargs), but I think this function can come in very handy in other functions (esp. plotting functions) as well in future. So, it would be nice if you could move it to the mlxtend.utils :)

[rasbt]

I don't have a better name at hand, but under a more general name like "format_kwarg_dictionaries" or so?

[jrbourbeau]

Sounds good to me! I have a couple of quick questions:

1. Should this function go into its own module in utils, maybe plotting.py, or into one of the existing modules like checking.py?

2. Should I also add the leading underscore to make this function private in utils (i.e. _format_kwarg_dictionaries vs. format_kwarg_dictionaries)? I'd imagine this function will only be used for internal stuff and not directly by users, so the leading underscore would make sense.

Thanks!

[rasbt]

Good points! I think we could put it into checking, because it's somewhat related to checking kwargs. I wouldn't suggest making it private, because it's generally useful (how I think about private things is that these are more like helper functions that are not useful outside the scope of a particular function and more like a side effect of refactoring, whereas the public ones are more broadly useful).

[jrbourbeau]

I can also add a plot_decision_regions user guide example to illustrate how to use the style kwargs

[rasbt]

I can also add a plot_decision_regions user guide example to illustrate how to use the style kwargs

Thanks, that would be super nice :)

[jrbourbeau]

Unless you have any other comments, I think this PR should be good to go!

[rasbt]

Based on the code, this looks great, thanks so much! I will take a look at the notebook as well and give it a try, but it looks perfect so far!

[rasbt]

Looks and works perfectly, thanks for the great contribution!