Finish decomposition explained variance visualizer

[bbengfort]

Thanks to @georgerichardson we have an explained variance visualizer for PCA decomposition. This needs to be wrapped up as follows:

Note to contributors: items in the below checklist don't need to be completed in a single PR; if you see one that catches your eye, feel to pick it off the list!

• expressiveness for other explained variances
• labeling and key values features
• tests
• documentation

See #238

[bbengfort]

See this blog post: A One-Stop Shop for Principal Component Analysis for a detailed and readable explanation of PCA as well as the use of the "scree plot" for selecting the number of components, which is what this visualizer is largely for.

[bbengfort]

@naresh-bachwani let's make sure that we close this issue when you finish #954