[R] Passing a LaTeX length to Sweave

[Thomas Levine]

I'd really love to do this

\setkeys{Gin}{width=\columnwidth}
<<label=something,fig=TRUE,echo=FALSE,width=\columnwidth,height=0.3\columnwidth>>=
plot(y~x)
@

The \columnwidth macro does not work, of course, in the second line.
What can I do instead?