As @rawr said in the comments, your best chance is with mtext
:
plot(1, type="n", yaxt='n', xaxt='n', xaxs="i", yaxs="i",
xlab="xlabel", ylab="", xlim=c(0, 0.4), ylim=c(0, 1))
mtext(expression("axis "*italic(r[infinity])),side=2,las=1,line=1)
The reason why you can't do that directly from plot
is that, internally, plot
uses title
to draw those labels and title
(for some reason) doesn't take las
or srt
as arguments.