Currently it is just added to MainWindow's centralWidget.
What do you mean added? Are you calling QMainWindow::setCentralWidget() anywhere? Or are you calling it more than once with multiple widget instances?

Have you "previewed" the MainWindow UI in Qt Designer? My guess is that you actually haven't added the QMdiArea properly in Qt Designer and it is not actually being set as the central widget. But you haven't posted the UI file so there is no way to check that code.

Have you tried adding a subwindow with a different widget? Like a QTreeView or QPushButton or something else? (Because they have a minimum size). If that works, then you may have to give the QwtPlot a size before the sub window can be shown properly.