Fixed aspect ratio QwtPlotSpectrogram

Hello all,
I am plotting some data in a subclass of QwtPlotSpectrogram.
The spectrogram is inside a QMainWindow along with some plotting 
controls below it.
I want the aspect ratio of the Spectrogram to stay fixed even when the 
user resizes the MainWindow.
Currently the aspect ratio starts out wrong (the spectrogram is wider 
than it is tall even though the y range of the data is larger than the x 
range of the data), and can be changed to whatever by resizing the 
mainwindow.

How do I set the aspect ratio to be correct and keep it that way even 
though the user tries to resize the Mainwindow?

Thanks in advance for any answers!
Andreas

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