There is an iPhone app called Bang Bang that does just this. It has warts[1], but overall does the job. If I were doing it I'd use the HTML-5 localStorage and

There is an iPhone app called Bang Bang that does just this. It has warts[1], but overall does the job. If I were doing it I'd use the HTML-5 localStorage and the 'manifest' offline web page stuff and just keep it all on person's phone.[2] Maybe I'd sync it up to a server somewhere in case they lost their phone. [1] Warts list: • Bang Bang is inexplicably slow. I'm down the stairs, through the house and out the door sometimes before I can get my weight entered. • The keyboard entry is inappropriate. Even assuming you want keyboard, you only need 10 digits and a decimal point, the whole keyboard makes the touch points to small and fiddly. Still better would be a roller. Today's weight is pretty close to yesterday's. One motion would suffice. • Some error checking wouldn't be bad. If I accidentally enter 600 pounds more than yesterday, maybe flag me instead of just graphing the result. • The starting date is lost in the UI. • The UI to get the starting weight and ending targets is unintuitive and will only be found by accident. Like by tapping at the screen while yelling "Why is this tiny graph so slow to draw‽"