Fix plotter not doing output transformation for fields

[jonathan-eq]

Issue
While doing #12967 I noticed plotting mean values for conductivity was in the negatives when running heat_equation. This is not possible, and it turned out we dont do transformations when plotting fields.

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')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.63%. Comparing base (d005912) to head (2fc3a00).
⚠️ Report is 24 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13046      +/-   ##
==========================================
+ Coverage   90.55%   90.63%   +0.08%     
==========================================
  Files         454      454              
  Lines       31233    31257      +24     
==========================================
+ Hits        28282    28330      +48     
+ Misses       2951     2927      -24     

Flag	Coverage Δ
cli-tests	37.30% <42.85%> (+0.02%)	⬆️
gui-tests	68.14% <85.71%> (+0.11%)	⬆️
performance-and-unit-tests	77.22% <57.14%> (+0.10%)	⬆️
test	46.21% <42.85%> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jonathan-eq]

ln(0.6368) = -0.4512, so this checks out.

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing jonathan-eq:05032026_fix_stddev_plotter (2fc3a00) with main (c609cb1)}

[xjules]

Good one! 🚀

[jonathan-eq]

@larsevj and I discussed this, and we are not sure what is the expected behavior for the plotting. Should we plot with the transformation, or without. Maybe we should even do a checkbox to toggle between them. Maybe @dafeda or @oyvindeide knows.

[dafeda]

@larsevj and I discussed this, and we are not sure what is the expected behavior for the plotting. Should we plot with the transformation, or without. Maybe we should even do a checkbox to toggle between them. Maybe @dafeda or @oyvindeide knows.

I suggest talking to users. Perhaps ask in #ert-users as a start.

[xjules]

@larsevj and I discussed this, and we are not sure what is the expected behavior for the plotting. Should we plot with the transformation, or without. Maybe we should even do a checkbox to toggle between them. Maybe @dafeda or @oyvindeide knows.

I suggest talking to users. Perhaps ask in #ert-users as a start.

Given that we plot only transformed values for scalar parameters I think this should be the default here as well (in case there is a transformation specified ofc).
Asking a question is good idea regardless.