[campersand]

You can already do this actually. In the highlight settings window, just change the draw mode from freeform to line or rectangle.  See which one you like more.

Hey, thanks for the response! I should have been clearer in my original message. I am using the rectangle highlighter, which makes the lines straight, but they can still be slanted across the pages if I'm not careful. What I'm saying is that it'd be great to have an option to force all highlighter lines to be completely horizontal.