pgup/pgdown in R Graphics Window under Linux

Dear S-users.
This should be an easy one: How do I change pages on an X11 graphics
device under linux?

I thought that the page-up/page-down keys were supposed to do the trick,
but the frame (window) seems to be kind of immune to any kind of keyboard
input. The only reaction I ever see is that the mouse pointer changes to a
"+" when moved into the frame.

I issue these commands:

library(lattice)
xyplot(speed~dist|speed, data=cars, layout=c(3,3))