composed plot improvements

[gtduncan]

Describe your changes

Made each curve in a composed plot distinguishable with a dashed line for one and a solid for the other. Additionally, the legend was resized in order to allow the user to better see the plot.

Issue ticket number and link

Closes #237

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have added thorough tests (when necessary).
• [ ] I have added the right documentation in the docstring and changelog (when needed)

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:books: Documentation preview :books:: https://sklearn-evaluation--263.org.readthedocs.build/en/263/