My jQuery knob plugin called knobRot has relative controls. It uses sprites rather than canvas, and there are a few other issues in the tracker, but you can see a demo here: http://www.domitable.com/static/side-projects/jquery-plugins... and this is the source on GitHub https://github.com/AlexanderParker/knobRot

I also would like to have a way of locking the mouse position, but sadly that would give black hat developers a nice exploit (for harvesting likes etc).