What normally happens in a CAD program is that if you use shift, the lines are constrained to to 30, 45, 90 and multiples of such. I don't quite like this, but it's common practice, I would rather see the accepted mode universal with the ability to toggle the reverse being true as a set-up option. Doesn;t one normally draw "straight" lines or a triangle with "nice" sides like 45, 45, 90 So, you make the line drawing tool so it acts like a CAD program and "straightens" the line when shift is pressed. If you enable some option, then drawing lines that are 0, 30, 45 and multiples of such would be the unshifted mode of operation. What would also be helpfull is a set of inputs that are dynamic. e.g. when drawing, you can set the length, angle etc. Usually these are placed at the bottom of the page.