Add ratio plots

[matthewfeickert]

Look at ratio plot visualizations from coffea:

https://coffeateam.github.io/coffea/notebooks/histograms.html#Styling

and the docs for coffea.hist.plotratio.

[matthewfeickert]

@dguest also has some code that might be helpful for deciding how to implement ratio plots that look nice. He's used it to get Figure 5 for ATL-PHYS-PUB-2020-019 which looks very nice:

[matthewfeickert]

@dantrim probably also has code to borrow over at dantrimania: https://github.com/dantrim/dantrimania/tree/master/python/analysis/utility/plotting