paste_linter should not warn inside expression() in a plot

[mcol]

This suggestion is actually misleading, as only paste() and not paste0() is allowed when expression() is used within a plot, such as to generate title or axis labels:

warning: [paste_linter] paste0(...) is better than paste(..., sep = "").
      main = expression(paste("Plateau test ", L[n], ",", L[x], " curves", sep =
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The rationale is that only a certain number of functions and operators are valid inside expression():
https://stat.ethz.ch/R-manual/R-devel/library/grDevices/html/plotmath.html

[MichaelChirico]

Reading ?plotmath, is this the right fix?

Actually, AFACIT, sep= has no impact in plotmath, so there is no difference between expression(paste(x,y,z)) and expression(paste(x,y,z, sep="")). (If I'm wrong, could you please share a plot demonstrating the output difference?)

So maybe this should be resolved by instead recommending to just drop sep='' instead?