OmniGraffle is a proprietary diagramming application:
It has a custom file format based on the XML-based Apple plist format:
The extension of OmniGraffle files is .graffle and the files may be optionally gzipped.
There is a C library for reading Apple plist files here:
Here are a few samples available at the URLs below:
David, does adding support for this format sound okay to you? If so, please confirm the ticket.
I doubt I'm going to work on it myself, but, in general, why not... Btw, all texts are saved as RTF documents, which makes the task considerably harder (well, I can imagine requiring external support for RTF import, which would probably be reasonably doable in LibreOffice, but would complicate the integration anyway).