[9fans] plot(1) to file?

[sqweek]

 Hey guys,
 I've got some data I want to put in a graph. I got graph |plot going
easy enough, which gets me a nice pic on the screen, but I can't work
out how to generate an image file instead. plot(1) mentions troff
which convinces me that it is possible, I just can't figure out how.
 I'm using p9p, otherwise I could probably do something clever with
/dev/screen...
-sqweek