A Diagram is a special type of Architecture Management resource. Creating a diagram involves the same steps as creating a resource (its title, GUID, properties, etc.).
Additional steps are:
Register the diagram image
The diagram image is a snapshot of the diagram as displayed in the authoring tool.
It is assumed here the authoring tool provides a mean (e.g. an API) to capture the diagram image. If that's not the case, you should not create Diagram instances that would be useless without such important feature.
Create diagram areas
Diagram areas are nodes (rectangles) and edges (paths) displayed in the diagram that reference resources being published. Registering a diagram area enables the user to navigate to the corresponding resource by clicking on the area.
The best case is the authoring tool provides a mean (e.g. an API) to capture diagram areas coordinates. If that's not the case, you may still create Diagram instances, as this is considered important but not critical information.
Publisher Schema - Types and Resources
Publisher Schema - Diagrams