You create a scene and view and add the label to the scene, then tell the view to show itself with: -
view->show()
If, as you state, you want the QGraphicsView on the label, don't add the label to the scene, but add the QGraphicsView to the label: -
QLabel* pLabel = new QLabel(mainWindow); // setting mainWindow as the parent
QGraphicsView* pView = new QGraphicsView(pLabel) // set the label as the parent of the view.
You do not need to call show, as the label will handle that for you, assuming the label is on a Widget that is already displayed.
Now, instead of setting the pixmap on the label, create a pixmap in the scene with: -
pScene->addPixmap(pixmap);