a while ago (2 years) I was part of a team that built a POC data logger based on espruino which the company decided that they would remain with the old C codebase and not update, they went out of business.. however I wanted to post some of the things we learned :
Circular buffers : use this data structure
log : Store time but transmit reading as a offset to the current time i.e. 567 seconds ago 3000 seconds ago do not think your time is the same as the receiver even if you know better
graphs : use SVG to display graphs directly from the log or generate the bitmaps from the data each time, basically use the data rather than trying to do fancy rendering the users will render what they want in the format they want just give them the data.
data store : sometimes store high and low and average of the 24 hours is enough...
have fun with the lenovo watch's ;-)