[FIX] Plot axes: No SI prefix for values below 1 with pyqtgraph 0.14

[raashish1601]

Issue

Fixes #7272

Description of changes

With pyqtgraph 0.14, plot axes without units get an SI prefix for any range below 1, so a Scatter Plot of values between 0 and 1 shows (x0.001) and ticks scaled by 1000. pyqtgraph 0.14 added getSIPrefixEnableRanges, and for axes without units its default is ((0, 1), (1e9, inf)). pyqtgraph 0.13 instead skipped the m and k prefixes for such axes, so nothing was scaled between 1e-3 and 1e6.

StyledAxisItem (the base of Orange's AxisItem, used by Scatter Plot, Line Plot, Bar Plot, heat maps and others) now overrides getSIPrefixEnableRanges and returns ((0, 1e-3), (1e6, inf)) for axes without units, which is the 0.13 behaviour. Axes with units keep pyqtgraph's default. I overrode the getter rather than calling setSIPrefixEnableRanges once, because setLabel (called on every axis update) resets the ranges. pyqtgraph 0.13 never calls this method, so nothing changes there.

Tests: TestAxisItem.test_si_prefix_without_units checks the scale for ranges up to 0.5, 5e5, 5e-4 and 5e6. On pyqtgraph 0.14 it fails without the change (scale 1000 for 0.5) and passes with it; on pyqtgraph 0.13.7 it passes either way. test_plotutils.py, test_owscatterplot.py, test_owlineplot.py, test_owbarplot.py and test_owdistributions.py pass locally (Windows, pyqtgraph 0.14.0, against the Orange 3.40 wheel with this patch applied).

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.97%. Comparing base (eea7a75) to head (aec069e).

❌ Your patch check has failed because the patch coverage (75.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7337      +/-   ##
==========================================
- Coverage   88.98%   88.97%   -0.01%     
==========================================
  Files         337      337              
  Lines       74598    74602       +4     
==========================================
  Hits        66378    66378              
- Misses       8220     8224       +4     

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[raashish1601]

Added a test for axes with units, which covers the line codecov flagged. The "Scientific Python nightly wheels" job is unrelated: it fails with ~560 errors from the nightly pyqtgraph (LegendItem.layout is no longer an attribute), and it fails the same way on master (https://github.com/biolab/orange3/actions/runs/37914329734).