DataFrame.plot error when both 'color' and 'style' arguments are passed (GH9671)

[davidbrochart]

closes #9671

self.style can be a list of strings or a string. If it is a string, we make a list containing that string, so that we only have to deal with a list of strings. We check every element in that list and raise an error if it has a color symbol.
The test makes sure that calling plot() and passing both color and style arguments is allowed if there is no color symbol in the style string(s), and that passing both color and style arguments is not allowed if there is a color symbol in the style string(s). This test is done for a dataframe and for a series.

[TomAugspurger]

Thanks. In the future, you don't need to open up a new PR. You can push changes to your original pull request and github will pick them up. I made a couple comments inline. Please let me know if you've got any questions or if anything I wrote doesn't make sense.

The tests look great! I'm glad you included one that should raise an exception (I always forget these). I just had a few things to make them fit into our test framework.

Once you make those changes you can test these specific ones with nosetests test_graphics:TestDataFramePlots.<test_name>. You also might want to make new tests instead of attaching them to test_plot. That way if they fail in the future we can easily see which test failed.

[TomAugspurger]

Instead of wrapping the code in try / except plots, just call the function.

ax = df.plot(color=['red', 'black'], style=['-', '--']) and then make sure the color and styles match what's expected.
You can get the actual linestyle with result = [line.get_linestyle() for line in ax.lines]
Then compare that to the expected with self.assertEqual(result, ['-', '--'])

Same thing for the colors, [line.get_color() for line in ax.lines]

[davidbrochart]

Thanks for your comments Tom. I made the changes you suggested.

[TomAugspurger]

@davidbrochart could you rebase on master and add a release note in doc/source/whatsnew/v0.16.1.txt? Thanks.

You can also squash your commits down to one. Let me know if you have any troubles.

[davidbrochart]

@TomAugspurger I rebased on master and added a release note. Hope this is fine.

[TomAugspurger]

Merged via f00d6bb

Thanks @davidbrochart