[R] Producing png plot in batch mode

[Vittorio]

I have set up an R procedure that is launched every three hours by 
crontab in a unix server. Crontab runs at regular intervals the 
following line:
R CMD BATH myprog.R

myprog.R (which by the way uses 
R2HTML) should create an updated png graph  to be referred to and seen 
in an intranet web-page index.html.

The problem is that both:

png
(....) 
plot(...) 
dev.off() 

AND:

plot(...)
HTMLplot(...)

fail when 
launched in a batch manner compalining that they need an X11() instance 
to be used (I understand that they work only in a graphic context and 
intarictively).

How can I obtain that png file? 

Ciao
Vittorio