Add docstring and tests for ExplainedVariance visualizer

[AjaySinghAdhikari]

This PR fixes #issue_number (If you are fixing a bug) which reported a bug that caused a problem to occur when users...

(or if you are introducing a new feature) which requested a feature to allow the user to...

I have made the following changes:

Sample Code and Plot

If you are adding or modifying a visualizer, PLEASE include a sample plot here along with the code you used to generate it.

TODOs and questions

Still to do:

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Questions for the @DistrictDataLabs/team-oz-maintainers:

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CHECKLIST

• Is the commit message formatted correctly?
• Have you noted the new functionality/bugfix in the release notes of the next release?

• Included a sample plot to visually illustrate your changes?
• Do all of your functions and methods have docstrings?
• Have you added/updated unit tests where appropriate?
• Have you updated the baseline images if necessary?
• Have you run the unit tests using pytest?
• Is your code style correct (are you using PEP8, pyflakes)?
• Have you documented your new feature/functionality in the docs?

• Have you built the docs using make html (must be run from docs/)?