Pretty much the same way as described in this tutorial:
Only instead of creating a Line sprite, you draw directly on canvas using "Line" action.
You can also draw by creating sprites and pasting them on the canvas if you need a "fancy" line (multi-colored, blurred etc.)
I have tried this and can't get it to work with the touch plugin. As the canvas line action takes four values X1, Y1, and X2, Y2 what do I set these too?