You will probably have to convert your data to a regular x-y grid (i.e. equally-spaced in x and y) and use bilinear or some other form of interpolation to create z values from the values at each corner of the cell.
I believe it is possible in Qwt 6 to create an abstract data source which will furnish the data to the plot without having to make an interpolated copy. See QwtRasterData. The data source basically maps your actual data via interpolation into the rectilinear grid required for the spectrogram. I am sure Uwe can provide a definite answer.
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