A new rcParam legend.linewidth has been added to control the line width of
the legend's box edges. When set to None (the default), it inherits the
value from patch.linewidth. This allows for independent control of the
legend frame line width without affecting other elements.
The .Legend constructor also accepts a new linewidth parameter to set the legend frame line width directly, overriding the rcParam value.
.. plot::
:include-source: true
:alt: A line plot with a legend showing a thick border around the legend box.
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], label='data')
ax.legend(linewidth=2.0) # Thick legend box edge
plt.show()