I guess you could run an event every tick (each frame) where you save the position of the touch to variables (varX1, varY1, varX2, varY2) and use more variables for distance info. Touch.X and Touch.Y expressions combined with distance(x1, y1, x2, y2).
Something like this...
set varX1 to varX2
set varY1 to varY2
set varX2 to Touch.X
set varY2 to Touch.Y
set varDistanceChange to distance(varX1, varY1, varX2, varY2)
set varDistance to varDistance + varDistanceChange
set varDistanceChange to 0
Then based on the distance travelled (varDistance) decrease the thickness of the canvas drawing line.