Automated text coloring in plot function

[dbabbitt]

This takes the background color of each rectangle and decides if the text would look better being black or white, and sets it accordingly.

[laserson]

Thanks for the contribution @dbabbitt! A lot of this diff seems to be unrelated to the new functionality you're proposing. Do you think you could remove those unrelated changes (e.g., formatting). Also, please run the code through the black formatter first.

[laserson]

Closing this as stale