The cache of the canvas is intended to speed up painting the content of the canvas for situations, where the content hasn't changed ( comparable to the backing store, but tailored for the situation of the Qwt framework ). But the cache is not intended for exporting the plot, what is pretty obvious as you will never see any axes.

Instead you have to use QwtPlot::print what is a much more powerful solution ( even comparable to QPixmap::grabWidget() ) as you can create scalable vector graphics in formats like SVG or PDF.

Uwe