I wish I had this rich of a data set! I'm curious how you all do retrospective weight tracking if you haven't kept a log like Tafkas (sorry don't know your full name)? Do you have a rough sense for the weight and can ascribe it to a given month? It seems like whenever I take my weight it is through a mental note.
Qualitatively I've been using my own facial image as a way of determining relative BMI / body fat percentage. While imperfect, there has been some published research showing that this link is possible (face image to BMI) through machine vision and careful training of ML models.
I gave a presentation on the topic at the last New York Quantified Self show & tell, happy to share if anyone is interested, happy to share early prototypes and the deck (video should be up in the next month or so). Starting to develop this into a more complete product, more information here - http://www.enfluence.io/