Allow x-labels

[michaelweylandt]

From class: How can we put x-labels on graphs?

My reply: Not sure how to handle in the general case, e.g.,

library(xtsPlots)
X <- xts(matrix(rnorm(250), ncol=5), Sys.Date() + 1:50)
plot(X, layout.screens=matrix(c(1,2,4,5,3,3), ncol=2, byrow=TRUE))

But maybe there's a way to handle in the simple case?

Note that mtext doesn't seem to work for the one panel case.