Qt Quick 2 uses a scene graph for efficient rendering on the GPU. Unfortunately this makes it impossible to embed classic widgets into the scene. The old approach to embed such widgets with the help of QGraphicsProxyWidget works only with Qt Quick 1, because internally it uses a QGraphicsView for all the heavy lifting and QGraphicsProxyWidget is meant to be used with it.
As of now there are no plans to enable embedding classic QWidgets into the scene graph I know of. I think this is rather unlikely to change, because the concepts of QPainter, the painting framework used for the classic widgets, and the new scene graph doesn’t play well with each other.
There some efforts to develop new widgets sets specifically tailored for the needs of QML, but none of them are as powerful and mature as the classic widgets. The most prominent ones are the QML Quick Controls, bundled with Qt since version 5.1.
If you really depend on QWT my advice would be to stick with Qt Quick 1.1 for now. It’s still bundled with Qt 5, probably for cases like yours. That way you won’t take advantage of the new scene graph, though.