"Time at Pressure" question.

[michael9346]

Can someone please explain what the "time at pressure" graph means? I just noticed I could go back a month and it shows this large increase around the same time each night (11:30 to 12:30) I couldn't find anything on this particular graph in Sleepyhead for Beginners? Is it a glitch because I can't understand the same time every night. As always thank you in advance for your help. 

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[Walla Walla]

What your seeing is the pressure at 11.5 to 12cm. If you look to the left it shows how many minutes it was at that pressure.