I imagine you could keep multiple diagrams in a List<Diagram> collection, add a TabControl above the DiagramView, and switch active DiagramView.Diagram to respective collection element when TabControl's active tab changes.

If these diagrams are related to each other and should be saved together, take a look into DiagramDocument and TabbedDiagramView classes, shown in TabbedView example.

The hidden diagram is used just for custom-drawing the items in the ListBox on the right. You could probably replace that listbox with a NodeListView instance.

Regards,
Slavcho
Mindfusion