if/else for plot/lines?

I'm interested in writing a function that constructs a new plot on the
current graphics device if no plot exists there yet, but adds lines to
the existing plot if a plot is already there.  How can I do this?  It
seems to me that the exists() function might be co-opted to do this, but
it's not obvious how.

Many thanks,

Roger

Roger Levy                      Email: rlevy at ucsd.edu
Assistant Professor             Phone: 858-534-7219
Department of Linguistics       Fax:   858-534-4789
UC San Diego                    Web:   http://ling.ucsd.edu/~rlevy