History Pulse Ox graph

runny said:

do you know what the grey bar below 0 means? 

The grey bar is the altitude. It looks like there was some deeper dip at the beginning of the week. Possibly because the watch was submerged in water, or due to some improper altimeter calibration, dirt in the pressure port sensor, strong wind, or another event causing unusually high pressure reading (high pressure = low altitude).

Thank you for your answers. In fact now the strange line below 0 disappeared but the grey bar is still there. The width of the grey bar is about 2k, and in negative altitude, I wonder why is still there if there's no case this last week to be gone below 0. 

Most likely because the minimal value calibrating the scale is stored longer time than just a week (or perhaps it even may be permanent)

Well, thanks!