display to external device

However, I usually connect to the Linux box via ssh from a machine  
running
MacOSX using x11. When I run R remotely like this, is there a way to  
output
the display from R on the linux server to the x11 on the MacOSX  
machine or
have x11 on the mac launch quartz to display the plot from the linux  
server?

Try to log in with ssh doing
ssh -X server

This should redirect X11 through the ssh connection. Then plot should  
open an x11 window, or you can open a display manually by entering  
x11() in R.

You can check that the X11 redirection works by trying to launch  
something like xterm from the linux box after you ssh to it, and see  
if the xterm window opens on the mac.
The mac needs to have XDarwin installed, which it usually has.