Problem with Zoomer

Hello,

I ran into a problem with the zoom function applied like in spectrogram example.
As soon as I click the right mouse button to zoom back again, then all plot items appear as tiny objects in the left bottom corner of the QwtPlot. I can then zoom in by dragging a selection rectangle around those tiny objects and after several zoomin in, it is possible to zoom out correctly by using the right mouse button, but only until the last saved zoomed action, where everything is again in the left bottom corner of the plot as tiny objects.
Could you tell me how to avoid this failure behaviour? Thank you.

best regards,

Vitali