Enable Storage of Histogram/ROC Objects

[afroch]

Summary

This pull request introduces the following changes

• Histogram Refactor: Binning/Normalisation is now executed once during Histogram construction instead of every draw call, making plotting faster and keeping per-instance state consistent.
• Persistence Layer: Adds save()/load() logic (eventually moved to PlotLineObject) so any histogram can be serialised to JSON/YAML and reconstructed later, including NumPy arrays and styling tuples.
• ROC Curves: ROC curve objects can now be saved/loaded, similar to Histogram objects.
• Support for High-Level-Plots: Support for high-level plots with automatic saving/loading of Histogram/ROC objects. Only for Results for now. AuxResults will need bigger changes.
• Updated Plotting Flow: Introduces a new “histogram plotting order” and propagates it up to the high-level plot helpers.
• Docs + Examples: New example script shows how to store and reload histograms; corresponding user-guide pages and the changelog are added.
• Robustness: Fresh unit tests cover the new code

Migration notes

• Existing plot calls continue to work; however, if user code relied on lazy histogram construction it should now create Histogram objects first and re-use them.
• Saving/loading is opt-in; nothing is written unless save() is invoked.

Overall, this PR modernises histogram handling by treating them as first-class, serialisable data objects while trimming redundant per-plot computation.

Conformity

• Changelog entry
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.34%. Comparing base (831d922) to head (0aedc9e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #319      +/-   ##
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+ Coverage   98.14%   98.34%   +0.20%     
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  Files          53       53              
  Lines        5711     5922     +211     
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+ Hits         5605     5824     +219     
+ Misses        106       98       -8     

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[dguest]

Maybe adding noise, but at some point we might want to look at this other effort toward a histogram storage format: scikit-hep/uhi#105. Seems a lot more specific about just storing the data, without the plotting code, but that would be enough to speed our plotting up a lot.

[samvanstroud]

Maybe adding noise, but at some point we might want to look at this other effort toward a histogram storage format: scikit-hep/uhi#105. Seems a lot more specific about just storing the data, without the plotting code, but that would be enough to speed our plotting up a lot.

We need to support other kinds of plots too though :( Not sure if we want to combine a third party solution with an in-house one?

[afroch]

I think we can discuss other formats but the issue I fixed in this PR is a general issue. So therefore I would delay adding new third-party solutions to later

[dguest]

yeah I'm not saying we should delay this. I guess better to make this into an issue right?

See #322.