The key thing is the function bquote().
E.g.
> N <- 5
> xxx <- bquote(bold("Figure"~.(N)~"Plot"~C[max]~"versus CrCl"))
> plot(1:10,main=xxx)
(Not sure about the ``bold'' business, but R doesn't object.)
Or to take a simpler and clearer example:
> N <- 5
> plot(1:10,main=bquote("Figure"~.(N))
cheers,
Rolf Turner
rolf at math.unb.ca