Eric,
I agree this is an issue (one of the particularly annoying), but I would suggest a different solution. So, the primary purpose of the "FSGraph > Save as" option is to save an opened graph into a new file, and .grf extension is appropriate in this case. I think that the PNG export feature is misplaced in the "FSGraph > Save as". We should add "Export" option in "FSGraph" menu and implement it properly. We should also include possibility to select some PNG export options like resolution, scale, exporting area, etc.
Regards,
Nebojša