PR summary

This pull request fixes issues with the grouped_bar method in Axes related to legend labels, bar colors, and hatch patterns.

Why is this change necessary?

Previously, single labels passed to grouped_bar were not applied to all bars, resulting in empty legends. Additionally, color and hatch patterns were not consistently applied across all grouped bars, and passing label and color twice could cause a TypeError.

What problem does it solve?

Ensures that a single label string propagates to all datasets.
Correctly applies colors and hatch patterns for each bar.
Prevents TypeError when labels or colors are passed multiple times.
Updates tests to verify proper handling of single and multiple labels.

What is the reasoning for this implementation?

By expanding single labels to a list internally and correctly mapping colors and hatches to each dataset, we preserve intuitive usage of grouped_bar and maintain correct legend, color, and hatch behavior without changing the API.

Example usage:

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
x = np.arange(2)
heights = [np.array([1, 2]), np.array([2, 3])]
ax.grouped_bar(
    heights,
    positions=x,
    labels=["dataset1", "dataset2"],
    color=["C0", "C1"],
    hatch=["//", "xx"]
)
ax.legend()
plt.show()

PR checklist

closes [Bug]: grouped_bar can pass label and color twice, causing TypeError #30739