Well, following a heads up from Simon Ubsdell I investigated this Nattress plugin and discovered that the onscreen curve line is just a representation of the controls in the inspector.
One might get the impression that you control the levels by manipulating the curve on the screen as you might do in photoshop but, unless there is a bug with the published software, this does not seem to be the case.
Simon Ubsdells free plugin however can be directly controlled in the viewer and works really well. There is link to it is in Craig Seemans post above.