Add ratio plot or residual plot tutorial

[ekauffma]

Ratio plots or residual plots are commonly used in HEP figures. It may be a good idea to include an example using mplhep (perhaps on one of the existing histograms with errorbars).

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[ekauffma]

episode 5 may be a good spot to add this!

[klieret]

Linking scikit-hep/mplhep#153 as mentioned by @ekauffma : mplhep doesn't seem to have anything built in to do this at the moment...

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.