Tracking user-entered data in wordpress

This is a little out there, not sure if this is the right place to ask… I want to be able to track trend data (specifically weight loss). The idea is that a logged in user will enter their weight loss from the front end, this data can be collated and show combined weight loss/gain over time. I don’t want to start with a weight an count down, just a simple date, user, difference.

and if I could intergrate into buddypress, all the better!

Any ideas on how I could go about doing this? I had thought of attaching it as a custom field to a post, and the user could add notes, but getting this in from the front end is the bit that I’ve not done before.

Hope someone is able to help.