`axis x line` directly sets `every axis x label`, not append

[muzimuzhi]

Currently, keys [xyz] label style append (/.append style) every axis [xyz] label styles, while keys axis [xyz] line directly set (/.style) those styles. This makes the order of keys matters, see https://tex.stackexchange.com/q/611575.

For example, with x label style=red, axis x line=center, the red setting is lost. https://github.com/pgf-tikz/pgfplots/blob/3bc2f42258fbfac9fe50b2978459da7e76fc046c/tex/generic/pgfplots/pgfplots.code.tex#L2508-L2510 https://github.com/pgf-tikz/pgfplots/blob/3bc2f42258fbfac9fe50b2978459da7e76fc046c/tex/generic/pgfplots/pgfplots.code.tex#L2885-L2903

Proposal: In axis [xyz] line, use /.append style as well.

[muzimuzhi]

Should these every xxx styles be always appended, not overwritten?

Similar places: https://github.com/pgf-tikz/pgfplots/blob/3bc2f42258fbfac9fe50b2978459da7e76fc046c/tex/generic/pgfplots/pgfplots.code.tex#L892-L895 https://github.com/pgf-tikz/pgfplots/blob/3bc2f42258fbfac9fe50b2978459da7e76fc046c/tex/generic/pgfplots/pgfplots.code.tex#L912-L918

Off-topic: It seems what really wanted here is a dictionary/mapping data structure, not constantly growing comma-separated list, nor linear-time consuming latex3 property list (plist).