Diagonal straight lines with brush tool

I'm working on a project for a class and we need to animate shapes. However, we're not allowed to use anything other than the brush tool to draw (no shape tool, no pen tool). We're not allowed to have object mode on either. It's been a significant struggle for me to draw neat straight lines. I know that holding shift while drawing creates straight vertical or horizontal lines. Is there any way to do this with diagonal lines?