Automatically saving document backups
When you turn on the backup function in the General preferences, OmniGraffle keeps a backup version of each document as you work on it. In the case of a crash, power failure, or other nasty event, the backup version remains on the disk. When you reopen the document, the last auto-backed-up version appears as a document with unsaved changes. You can continue working with the backed up version, or you can choose Revert To Saved from the File menu to go back to the saved version.
If you close a document and choose not to save it, the backup version is discarded.