How to get selected nodes data and event through mouse selection ?
The standard configuration of each Diagram has a DragSelectingTool that allows the user to draw that magenta box to select the nodes and links within the rectangle upon mouse-up. GoJS Tools -- Northwoods Software and DragSelectingTool | GoJS API
You can then get the selected parts in the Diagram.selection collection. Diagram | GoJS API and GoJS Collections -- Northwoods Software