Why do I get black graphs in Publish?

My script used to work well, publishing a set of graphs. I have made some major changes and now when I run roughly the same "publish" script I get the surrounding text right but the graphs are all black squares with no data inside. Any idea what is causing it? Thanks Ornit

[Ornit]

OK I will answer myself for whoever stumbles on this problem in the future. It seems publish will make black graphs if the workstation is locked.