Libra for Android helps you track your weight using The Hacker's Diet system
If you've ever read The Hacker's Diet, this graph should be instantly recognizable. Each point shows the weight for a given day, while the trend line lets you see if you're gaining or losing weight.
When I use my "main" PC, I have a homebrew solution for creating and maintaining this graph. But I've had to find a temporary solution for Android, and Libra is it.
It's a beautifully simple app, very true to the spirit of The Hacker's Diet. You can create a shortcut on your homescreen that brings you right into the data entry screen, so you just tap the shortcut every morning, feed in your weight for the day and hit OK, and then you get to see your progress (or lack thereof) on the graph.
The app remembers the previous day's weigh-in, which makes it easy to enter today's weight (as they're usually not too different). The graph is zoomable and scrollable, and most importantly, you can export the data to CSV so you're not locked into the app.
If you ever need to track your weight using an Android device for any period of time, Libra is one excellent solution.