[Chris]

Joel's statement that there isn't an existing .graffleshapes file isn't .... 100% accurate. To find the existing graffleshapes file, go to your Applications folder (or wherever you've installed OG) and right-click on OG (or control-click, if you don't have right-click enabled). Choose "Show Package Contents" from the popup menu. This will bring up a Finder window with a single folder in it called "Contents". Go into that folder and then into the "Resources" folder. The .graffleshapes file is in there; it's called "shapes.graffleshapes". I'd suggest making a copy somewhere else to look at. You can open it in text edit; it's just a plain text file. All the shapes are in there, including the arrowhead shapes for the lines. Just search for "arrowheads" to find them. I've never played around with the arrowhead shapes, so I can't guide you from here. But I would guess you can add new arrowhead shapes by adding them to this file.

Just make sure you keep a copy of the original shapes.graffleshapes file around in case you screw up (you will, if you play around) so that you can revert to factory conditions.