Looks like the svg paint engine ignores clipping at all. Looking into Qt 4.5.0 beta I can't see any new code, so it doesn't seem to be on the TODO list of the Qt developers.

If you enable QwtPlotCurve::ClipPolygons, the curves are clipped against the paint device boundaries ( QPainter::window() ). Then you have to patch qwt_plot_curve.cpp, so that the canvasRect is used instead of painter->window().

Uwe