How can I set the spacing of a fixed axis using Qt / QwtPlot?

I want to have a graphics widget in my Qt application. Qwt provides such a QwtPlot widget. However, I cannot find a way to display a specific portion of the full range of my data.

Specifically, I want to display spectra with a frequency range from 0 to 2 ^ 14. However, for the GUI, only the audio range from ~ 20-20 kHz is important, so I only want to display this part.
Do you know of any way to tell QwtPlot about this?

Thank you for your responses.