This has been bugging me forever, and I finally figured it out. If you have a Wacom Intuos tablet (or maybe other types too) and the tracking seems off — drawing in a diagonal direction creates wavy or undulating lines, like the blue lines above — here’s how to fix it.

Take off the fingertip button; underneath, in the center, is a little tiny screw. Use a jeweler’s or eyeglass repair screwdriver to turn it. Just turn it one way or the other until it fixes the problem. I don’t think it’s strictly geared to the turning: turning it back where it was didn’t make it worse again. But the action of turning seems to have fixed it.