Well it depends on how you measure how much line has been drawn. Is it the length of the line or the time that the player has been drawing the line. If it's the first, well I'm not sure. That's a tricky problem. If it's the second on the other hand, you can have an expression like this.
Set *bar* size
x = bar.x * ([Max Time Player can draw line]-[Time player has been drawing line])
y = bar.y