I found my mistake yesterday, and thought I'd sent a reply to you. This site can be confusing - my reply didn't go. I'm still having problems: "Your submission could not be processed because the token has expired" Reloading the window doesn't help.
The error was embarrassingly simple, and unrelated to QwtPlot and QMdiArea. Because the infoFile was not found, file.open() returned false, and the program returned without reaching the QwtPlot code. My stupid oversight. Thanks to your simple code example, which omitted that leftover code about the info file, I was able to see my mistake - finally. Please accept my thanks - and my apologies.