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Raise a clear error when grouped_bar gets singular label/color (#30739)#32006

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Kundhan73:fix-grouped-bar-label-color-30739
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Raise a clear error when grouped_bar gets singular label/color (#30739)#32006
Kundhan73 wants to merge 1 commit into
matplotlib:mainfrom
Kundhan73:fix-grouped-bar-label-color-30739

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Closes #30739.

Axes.grouped_bar passes each dataset's label and color to bar() itself and also
forwards **kwargs. If a caller passes a singular label=/color= (they land in
kwargs), bar() receives the keyword twice and raises an opaque
TypeError: ...bar() got multiple values for keyword argument 'label'.

This adds an early guard in grouped_bar that raises a clear TypeError pointing the
user to the plural labels/colors parameters, plus a parametrized test.

Open question: the reporter expected label="dataset" to work (i.e. alias singular →
plural). I went with a clear error since a single label/color is ambiguous across N
datasets and contradicts the documented plural API — happy to change to aliasing if you
prefer. No changelog entry added since grouped_bar is provisional and 3.11 is
unreleased; let me know if you'd like one.

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[Bug]: grouped_bar can pass label and color twice, causing TypeError

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