Okay, here is one graph. It shows the possible calibration curve that might be used to calculate the charge of battery from the battery voltage.
The black dots are only the non-charging situations. The red dots are the heavy voltage drops like the well known 4%-drops. Since the data has a temporal resolution of around 5 minutes, I had to include the charging situations as well to get these drops, because if a drop happened, I instantly connected the charger to still use the phone.

Interesting part: Curiously, these drops in charge of battery don't come together with voltage drops, as you can see in the image. So, either the voltage sensor inside the N9 is not very precise or (most likely) the voltage is not the only indicator for the charge of battery...

Greetings,

NobodyInPerson

Edit1: I don't understand why the image gets resized so ugly... I payed attention on the maximum dimensions and file size...
Edit2: Okay, use postimg.org... that's it...