Add an option to sum over/underflow with the adjacent visible bins

[pieterdavid]

I didn't see any mention of this in the documentation, although it is reasonably common, and I think the plotting layer is the right place to do this (it seems more like a workaround elsewhere). Would a simple flag to histplot cover all common cases? Any preferences on naming? show_overflow is short, but may be found confusing... if I find a bit of time I'll try to implement this.

[andrzejnovak]

I've been wondering about what to do with underflow/overflow for a while, especially what would be the right way to visualize it. I think we don't have to deal with the trivial case histplot(h.to_numpy(flow=True)...) but it would be good to have for hist or TH1s going in. Some thoughts I have are these:

• by default, there should be some indication if there is flow, maybe an arrow?
• I prefer shorter kwargs, flow seems to be used already in some context
• Seems to me we have 4 cases
  • Nothing
  • Indicator
  • Sum
  • Display a new bin (maybe with a gap from the rest of the histogram?)
• arguments could be
  • None/False
  • "hint" (default?)
  • "sum"/"add"
  • "show"/True