Why not just have a normal straight line drawing tool like every other drawing program? The ruler thing is an amusing novelty, but totally impractical for quickly drawing straight lines.
do as in other apps. draw a shape that is either curved, circle rectangle or nearly straight line and hold in the end of the shape and that automatically makes the drawing a perfect curve / circle or straight line but still keep it as a drawing that can partly be erased. (please test pro-create a bit and you will understand what i mean)
Doug Farley commented
Also adding modifiers for snapping lines at 45-degree angles like you can in PowerPoint/Word/Visio with Ctrl to quickly create (and on the PC/Web).
Rectangles as well