[john.gersh]

I don't know if there's a primer, but the just looking at the XML probably tells one enough for extracting nodes and edges. The overall format is an Apple plist; as usual, the link to the DTD for that is in the file header. The XML is pretty much self-documenting; the names of the various elements and properties are self-explanatory.

The important element is the GraphicsList, an array of plist dictionaries, one for each object.

Lines that connect shapes have head and tail properties whose values are the IDs of the shapes connected. If the drawing is just nodes and edges that's pretty much all you need.

User-defined properties for an object might be helpful, too. The Note tab in the Properties inspector lets you enter key-value pairs. Those show up as the UserInfo property of the object in the graffle file. That might help you add easily-parsable information that your code can use.

As whpalmer4 said, you can also write an Applescript to extract the information you need and write it to some format convenient for your code.