Arbitrary axis interval without affecting the plot

[Uwe]

I have a QwtPlotSpectrogram attached to a QwtPlot, and I would like to scale the plot so that it spans the entire canvas.

You want to eliminate the margins at the ends of the scales, so that they are aligned with the canvas border ?

I would also like to arbitrarily set the axes regardless of how the data is on-screen.

Is this the question how to disable autoscaling ?

It would help if you try to use the terminology of the Qwt plot framework. Guess you mean your spectrogram item, when you write about "plot" otherwise a "plot that spans the canvas" doesn't make any sense at all.

Uwe