Latex and variables in plot label in R?

How do I use variables in Latex expressions in R?

For example:

plot(X, Y, main=expression(R^2))

Will put R with a nice superscripted 2 as main title.

But let's say I want it to say 'R^2: 0.5', with 0.5 coming from a R variable. How do I do that?

The hack of Owen is pretty cool, but not really the common way to do that. If you use bquote, this is actually pretty easy. bquote will do the same as quote, with the exception that everything between .() will be evaluated in a specified environment (or the global, if nothing is specified).

A trivial example :

X <- 1:10
Y <- 1:10
a <- 0.8
plot(X,Y,main=bquote(R^2 : .(a)))

Gives :

See also ?bquote and ?plotmath for more examples and possibilities

Well this works...

call(':', quote(R^2), a)

though it feels a little hacky to me since it's using R's : operator, whereas you just want to stick some text on the end. Maybe there's a better way?

tikz and psfrag allow you to use actual LaTeX code and output instead of plotmath's, resulting in better typographic consistency.

See this question for details. Getting LaTeX into R Plots

Another variation on @Joris' theme is substitute(). You give substitute() an expression and an environment or list within which to evaluate the expression. Passing a list is usually easiest, especially for jobs such as the one posed here.

plot(X,Y, main = substitute(R^2 : a, list(a = a)))

We can see why this works, by looking solely at the substitute() call:

> substitute(R^2 : a, list(a = a))
R^2:0.8

The a in the expression is replace with the value of a in the list.