In the top-right corner of every page you sould see links to sign up or login. When you click either link, you'll be taken to a Google Accounts page. If you already have an account with Google, you can enter the email and password there. Note that Google securely handles your email and password directly. WeighTracker is never sent your password. If you do not already have an email address registered with Google, you can click the "Create an account now" link on that same page to create an account with your existing email address.
You may notice that you cannot save any notes about a day unless you record a scale reading. The way the datastore is organized any days without scale weights are treated as empty. While it would be helpful to be able to flag diet lapses without weights, it is extra incentive to be consistent with your weight recording.
Editing old data can cause cascading recalculations. In order to limit these recalculations, data which is more than 30 days older than the latest entry becomes read-only. On a related note, if you skip 30 days between scale readings, no interpolation will be performed and the calculations will simply start fresh at the new date.
Your weights are only displayed when you are logged in. Other users cannot view your data or charts. Theoretically an administrator has access to all the data in the datastore. However, scale weights are recorded against system generated alphanumeric hash identifiers; users' names are stored separately in other tables.
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"Water is the most neglected nutrient in your diet but one of the most vital."
- Kelly Barton