[BUG] No `ylabel` unit label in `twinx`

[ivan-boikov]

Details

When adding a twinx plot with Unitful y values a corresponding unit label is not added.

using Plots, Unitful
plot([0,1]u"s", [0,1]u"m");
twinx()
plot!([0.1,0.5]u"s", [0.1,0.4]u"m")

GR example:

Backends

This bug occurs on ( insert x below )

Backend	yes	no	untested
gr (default)	x
pythonplot	x
plotlyjs		x (no units at all)
pgfplotsx		x (crash)
unicodeplots		x (no twinx?)
inspectdr		x (no twinx?)
gaston			x

Versions

Plots.jl version: 1.38.11
Backend version (]st -m <backend(s)>):

GR: 0.72.4
PythonPlot: 1.0.2
PGFPlotsX: 1.5.3
PlotlyJS: 0.18.10
UnicodePlots: 3.5.2
InspectDR: 0.4.3

Output of versioninfo():

Commit afb6c60d69a (2022-09-06 15:09 UTC)
Platform Info:
OS: Linux (x86_64-linux-gnu)
CPU: 12 × AMD Ryzen 5 3600 6-Core Processor
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-13.0.1 (ORCJIT, znver2)
Threads: 1 on 12 virtual cores
Environment:
JULIA_DEPOT_PATH = /home/ivan/.local/share/julia

[Ickaser]

There is an actual underlying problem here. But the given MWE creates an empty twinx and doesn't plot into it, so the lack of axis label is because there are no series on the second y-axis.

[BeastyBlacksmith]

Fixed by #5098