It would be really handy to be able to enable an ortho command that would limit the pen and highlighter to horizontal and vertical movement. This is used in AutoCAD programs, but it would be really helpful for those of us who cannot drag a mouse in a straight line!
The feature would be turned off by selection/deselection of a keyset or by use of a menu icon.
wow, nevermind. I found out what the ruler icon does!
The ruler gives you lines that are always straight, as you've found out by now. Unfortunately there's no preference for strictly horizontal/vertical lines. (There are some patches that do it, see under "ortho/snap" in the sourceforge patches tracker).
If you can draw lines that are kinda straight but not quite, you might also try the shape recognizer - which *does* have a preference for strictly horizontal/vertical lines over slanted lines. However, it is very possible that the shape recognizer is too hard to use with a mouse - that tool was really intended for tablet users, and expects you to draw a fairly good approximation of a straight line in the first place.