Notice at 140 minutes the two lines merge. This is what we dubbed the “death spiral.” The battery voltage has dropped low enough that the watch executes a brown-out reset. This restarts the code, which restarts the display, causing another brown-out…repeating until the battery gives out fully.
(For the record, battery life has been improved substantially since this early graph was taken, so you should get well more than 140 viewings from your watch!)
The stuttering parts of the line are due to the limited resolution of the Arduino’s analog-to-digital converter. Those could be filtered out by taking multiple voltage readings and averaging the results, but it wasn’t necessary to go to that level of detail here; seeing the trend was sufficient.