I take exactly one picture on the first of each month. Same bathroom, same overhead light, same unimpressed expression. Then I leave the folder alone for a while and don't open it until there are at least three shots in there.
Early on I'd pull up two photos side by side and spiral because one looked heavier than the other. Took me a bit to realize a photo is just a single weigh-in. One reading is weather, the trend is climate — same rule I use for the scale.
Speaking of which, I weigh daily. 0.2 up, 1.4 down, doesn't matter to me anymore. The 7-day average has been drifting down roughly 0.4 kg a week for the last four months. The photo folder is way lumpier than that. Month three looked identical to month one, then month five looked like a stranger.
If you're documenting this, keep the lighting boring and don't judge a single shot. Anyone else notice their camera roll lags behind their trend line?