BUG: fix boxplot column misorder

[zhuoyahuang]

• closes BUG: xticklables is not correctly set when using df.plot.box with group-by  #50427 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

the data.column is MultiIndex, for example

data.columns: MultiIndex([('c', 'value'),
        ('b', 'value'),
        ('a', 'value')],
       )

The problem with the original code is not respecting the order of by