How do I keep a drawing canvas overlaid on a viewport?

Hi

I have a vertically scrolling game, and I am using a Drawing Canvas on its own layer to draw lines based on TouchX and TouchY. It seems when my game scrolls upwards, I obviously need to keep the drawing canvas overlaid on top of the viewport, as my game currently scrolls beyond the drawing canvas and the drawing gets clipped.

I have tried turning off parallax, so it just remains in place, but this doesn't work either.

What is the approach to maintain the drawing canvas overlaid over the viewport, and still be able to interpret touch coordinates etc.

I think the answer is to turn off parallax on the drawing canvas, but use the ViewPortTop value somehow to manipulate any Touch Y co-ordinates. I am assuming Touch.X and Touch.Y are physical device co-ordinates (viewport) - or are they world X-Y coords?

Regards

SM