Problem

Currently, boxplots do not get legend entries.#27711 was an attempt to introduce them but only solved one very particular usecase and did not generalize well. See#27780.

There is alabels parameter, but that only sets x-tick labels. And it has one entry per box. For the legend we should have only one entry perboxplot() call no matter how many boxes that has, becauseboxplot draws N identically styled boxes and it does not make sense to have N identical handles with different labels.

Because of the x-tick relation oflabels and it's relation to individual boxes, this parameter is not suited for the legend labels.

Proposed solution

We need a separate parameter. There are basically two options:

Variant 1)legend_label: str

pro: simple and clear, no interference with the existing API
con: inconsistent with the rest of the library: all other functions uselabel for the legend entry.

Variant 2)label: str

pro: consistent with the rest of the library
con: Havinglabel alongsidelabels is quite confusing and easily leads to errors. (We could runtime-check for the type str vs list of str, to give helpful error messages but still ...)
If we want to go this way, we should renamelabels totick_labels or similar.