Visualizing flow bins

[andrzejnovak]

With protocol in (thanks @henryiii) operating on uproot/bh/hist objects lends itself to easily fetching flow bin contents. Should we think about providing a default to visualize them? Either directly or just with some kind of indicator if they are non-zero.

This would be different from just calling sth like histplot(hist.values(flow=True)) because in this case, they would show up as just regular bins. @nsmith- @matthewfeickert