"The Hacker's Diet" is an ingenious [book](https://www.fourmilab.ch/hackdiet/) by John Walker which applies a systems and signals view to bodyweight. If you haven't already, go read it.
Walker provides excellent online tools for tracking and Excel-/Palm-based tools for offline tracking. I'm not comfortable with either, though, and as it [turns out](https://www.fourmilab.ch/hackdiet/e4/pencilpaper.html), the calculations needed for trend calculation are simple.