Batterygraph relies on the data provided my the phone itself, so the values are as good as the data the OS uses itself.
When you switch off the device, obviously no data can be recorded...

There is no reason to switch it off or in flight mode for charging!

Also, most systems never charge up to 100%, because this is bad for the batteries. Usually it gets charged to 90..95%.
This is similar to any other device I had in my hands.

My device lasts on normal usage maybe 1.5 days. How ever I charge it every night to be on the save side. If you keep it in flight mode all the time (how useless this ever might be), it will last several days.