If using binary format, you could call Diagram.SaveToStream method to store the diagram as part of your own file. Alternatively store your data as a serializable Diagram.Tag object and SaveToFile will save it as part of the diagram file.

If using XML format, you could call Diagram.SaveToXml (XmlDocument) to save the diagram in an in-memory document, and then either move the root element to your own document, or add your own elements to the diagram's document.

Another option is to make your files zip archives and store diagrams' and your custom data separately inside.

Regards,
Slavcho
Mindfusion