Proper code

[gib]

I have a limited understanding of Qt and Qwt, as this post will reveal, and I apologise in advance for my ignorance.
Despite my poor grasp of it all, I have managed to get QwtPlots working in some fairly complex Qt programs. Now I am trying to do something much simpler, but following (so I think) the procedure that I have working in the other programs. But this simple example doesn't work.

I have attached the three files that make up the project, which I am building with Qt Creator 2.5.2 on VS 2012 with Qt 4.8.5 (64-bit). Qwt 6.1.3 is also built 64-bit. The missing component is mainwindow.ui, which I made by hand. It has a QMainWindow containing a QWidget (centralWidget), within which I placed a QVBoxLayout, and inside that a QMdiArea called mdiArea.

The program compiles and runs, but nothing I do makes it display the QwtPlot. mdiArea just shows as a grey rectangle. As far as I can see I am following my usual procedure, except that in the other programs centralWidget contains a QStackedWidget, and within that the vertical layout that holds mdiArea. I know that I am using an approach different from anything I've seen in the Qwt examples, but it seems to me that it should be close to working.

If somebody has the patience to look at my code and see where I am going wrong I would be very grateful.