In SVN trunk you find more OpenGL based options including a canvas derived from QOpenGLWidget.

Nevertheless don't expect too much, as the implementation is not yet complete and the quality of all OpenGL paint engines offered by Qt is not comparable to other backends. IMO the best and most performant backend for Qwt on the market is Qt4/X11 - hardware accelerated and without known bugs.

Concerning your question about QPainter: allmost all Qt chart packages will be QPainter based, simply because this is mandatory for creating PDFs. Not doing QPainter means, that you would have to implement 2 completely different backends or have to drop PDF or printing. Nothing that makes much sense beside you want to focus on specific use cases like an oscilloscope etc.

But even with the scene graph don't expect, that hardware acceleration will always be the best solution. Depending on the use case you will have different conclusions, when comparing CPU vs. GPU rendering. Better trust on good algos reducing what has to be painted - instead of challenging the hardware.

Uwe