This finds the DiagramPanel template child that becomes the value of Diagram.Panel. It calls InitializeLayers to create the various standard layers, it raises the TemplateApplied event so that you can initialize the Diagram.Panel's event handlers or establish data-bindings on its properties, it calls PartManager.RebuildNodeElements to create Nodes and Links from the model's data, and it calls LayoutManager.LayoutDiagram to eventually position the nodes if needed.

Reference

Diagram Class
Diagram Members
Base Implementation in OnApplyTemplate