Use matplotlib method for bin creation for histogram

[eilskra]

Issue
Resolves #13958

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[eilskra]

From visual tests it seems like it should be correct. Would love input on this @berland

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.38%. Comparing base (64bb156) to head (edddc2f).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13961      +/-   ##
==========================================
+ Coverage   91.34%   91.38%   +0.03%     
==========================================
  Files         477      482       +5     
  Lines       34062    34180     +118     
==========================================
+ Hits        31115    31236     +121     
+ Misses       2947     2944       -3     

Flag	Coverage Δ
cli-tests	36.03% <0.00%> (+0.06%)	⬆️
fuzz	43.63% <12.50%> (+0.10%)	⬆️
gui-tests	59.30% <62.50%> (+0.09%)	⬆️
performance-and-unit-tests	79.43% <100.00%> (+0.05%)	⬆️
test	46.12% <12.50%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/ert/gui/plotting/ert_plots/histogram.py	96.11% <100.00%> (+1.07%)	⬆️

... and 17 files with indirect coverage changes

[eilskra]

Think the initial is wrong

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing eilskra:histogram-bin-fix (edddc2f) with main (2a34262)}

[Copilot]

Pull request overview

This PR updates ERT’s GUI histogram plotting to use Matplotlib/NumPy’s built-in bin selection (bins="sqrt") rather than computing a single bin count shared across ensembles, addressing the reported issue where an ensemble’s histogram changed when other ensembles were toggled.

Changes:

• Remove global bin_count calculation based on the maximum ensemble size and delegate bin selection to Matplotlib (bins="sqrt").
• Rework log-scale bin edge generation using np.histogram_bin_edges on log-transformed values, and remove the custom _histogramLogBins helper.
• Update unit test expectations to match the new _plotHistogram call signature.

Reviewed changes

Copilot reviewed 2 out of 7 changed files in this pull request and generated 2 comments.

File	Description
src/ert/gui/plotting/ert_plots/histogram.py	Switch histogram bin creation to Matplotlib/NumPy’s "sqrt" rule; remove shared bin-count logic and the custom log-binning helper.
tests/ert/unit_tests/gui/plottery/test_histogram.py	Adjust test mock call expectations after removing the bin_count argument.

[berland]

Looking at the image diff for e.g. docs/ert/getting_started/configuration/poly_new/with_observations/coeff_b.png it is clear that the bin sizes / bin count is changed for iter-1, even though I cannot determine that from the code that it should have done so. This obscures visualization of what the bug entailed in this PR.

I wonder if we should ensure we fully understand what was wrong with the original code. Maybe by having a branch where both ways of computing bin edges are performed and printed to the terminal for qc, maybe also along with the data. Even better if the we could extract the old bin-making code to a function and prove it is wrong with a regression test, then fix it.

[ertomatic]

Screenshots differ from baselines. A baseline update PR has been prepared: equinor/ert-testdata#44