I would like to set the 'xlab' on the 'upper' x axis. The following code
shows roughly what I want by way of (one possible) expected behaviour.

par(mar=c(5.1, 4.1, 5.1, 2.1))
plot(rnorm(10), rnorm(10), xlab="the lower one")
axis(3, xlab="the upper one????")

There are various other ways that could be expected to work, but hopefully
the above is clear enough to show you what I want to achieve.

The following works, but is fragile (works without resizing the window of an
X11 device).

title(xlab="the upper one", line=-28)


Is there any better way to do it other than tinkering with 'line='?


Here is the same code as above but with comments just for reference.

# Give upper and lower margins an equal size
par(mar=c(5.1, 4.1, 5.1, 2.1))

# Plot (labeling the lower x axis)
plot(rnorm(10), rnorm(10), xlab="the lower one")

# Add the upper x axis (labeling fails!)
axis(3, xlab="the upper one????")

# Works but is fragile
title(xlab="the upper one", line=-28)

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