[senanurcetin]

Summary

Fixes #1667.

Fix inconsistent bar grouping when plotting Pandas DataFrames with a two-level MultiIndex.

Previously, df.hvplot.bar() grouped data by both index levels, while df.hvplot.bar(y="value") unexpectedly dropped the second grouping dimension.

Changes

• Preserve automatic MultiIndex grouping when y is explicitly supplied for bar and barh.
• Maintain the existing behavior when x or by is explicitly configured.
• Preserve heatmap behavior.
• Add regression tests for both bar orientations.

Verification

• The regression test failed before the fix and passed afterward.
• 800 tests passed, 6 skipped, 3 xfailed across the relevant hvPlot test suites.
• Ruff lint and Git diff checks passed.
• Only two files changed: 16 additions and 2 deletions.

AI Assistance

AI assistance was used for debugging, implementation, and regression testing. The changes were validated locally before submission.