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add itemboxalign option to legend constructor #29197

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add itemboxalign option to legend constructor #29197

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PR summary

This pull request adds the option "itemboxalign" to the Legend constructor. From the proposed announcement .rst:

Legend previously always aligns items using the "baseline" option, which results in
the appearance of vertical centering of the artist and label for multi-line labels.
This is sometimes hard to read. The introduction of the itemboxalign parameter to the Legend constructor
allows the user to change this behavior and choose a different desired vertical alignment.

import matplotlib.pyplot as plt

fig, ax = plt.subplots(1, 2)

ax[0].plot([5, 2, 8], label='long\nlabel')
ax[0].plot([4, 9, 1], label='another\nlong\nlabel')
ax[0].legend(title="align=baseline (default)")

ax[1].plot([5, 2, 8], label='long\nlabel')
ax[1].plot([4, 9, 1], label='another\nlong\nlabel')
ax[1].legend(title="align=top", itemboxalign='top')

plt.show()

image

I chose the name "itemboxalign" on a whim based on the existing rendering code in Legend. I think the name could be better, such as:

itemboxalignment
itemboxverticalalign
itemboxverticalalign(ment)
labelverticalalign(ment)

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github-actions bot reviewed Nov 26, 2024
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@timhoffm
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timhoffm commented Nov 28, 2024 •
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Thanks for the contribution. This looks fundamentally reasonable. I'm still thinking about proper naming.

Note: We unfortunately have an alignment parameter that only covers the horizontal alignment of title and content and is thus too generic and semantically overlapping the alignment here. But for backward-compatibility we cannot change that.

The new naming needs to be more specific, but itemboxalign feels very technical and it may be hard to guess what it is doing. Maybe handlealignment? If one wanted to be very precise handle_vertical_alignment, but that's quite long. Suggestions welcome.

@skkestrel
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I'll also note that in Legend documentation, the term "entry" is also used for the marker+label combination:

alignment{'center', 'left', 'right'}, default: 'center'
The alignment of the legend title and the box of entries. The entries are aligned as a single block, so that markers always lined up.

So, perhaps also entryalignment or entry_vertical_alignment - but I would argue that there's not much sense in talking about horizontal alignment of individual entries, so I slightly prefer the shorter entryalignment or handlealignment.

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