[FEATURE] Add ratio plot support to API

[matthewfeickert]

As there is already the ability in hist to add a ratio plot to the bottom of the main plot, as seen in the .plot_pull() API it would be great if the ratio plot functionality (which looks super nice and clean in the plot_pull examples btw 👍) could be added to the public API. If people agree this would be useful I'd be willing to contribute a PR.

Describe the problem, if any, that your feature request is related to

I don't have a problem per say, but just would like this feature to exist so that I don't have to implement it in a library that is already using hist. matthewfeickert/heputils#22

Describe the feature you'd like

I'd like to have the ratio plot functionality that exists already in the plot_pull API to be available through a public API that would be something like .plot_ratio.

Describe alternatives, if any, you've considered

None, as this would be up to the developers to decide if this is in scope or not.

[henryiii]

Sounds good. Plot_pull could call plot_ratio. Could be easier to upstream to mplhep too.

[matthewfeickert]

Sounds good. Plot_pull could call plot_ratio.

That makes sense to me

Could be easier to upstream to mplhep too.

I'll let @henryiii and @andrzejnovak decide where would be the best place for a PR, but at the moment it seems that

hist/src/hist/plot.py

Line 129 in 3827e75

def plot_pull(

is still mostly implemented in hist so I'll start work there until someone tells me to pivot.

[andrzejnovak]

Ideally, it would exist as a standalone function in mplhep such that one could also use it manually or with uproot TH1s, but since I'm not the one writing it I should shush :D

[henryiii]

I'd start by refactoring Hist's function and getting that out, then we can upstream it to mplhep and then move to using the new version here.