Selecting Object - Graph Subproject opens a response viewer tab and plots the analysis data of the subproject selected in the netlist. This command opens a new response viewer tab. If you wish to add the project to an existing graph, select that response viewer tab, then click on the Add button on the tool bar and select the project.
You may also launch the response viewer by clicking on the Graph button in the Launch tool bar when a subproject is selected in your netlist.