App data usage can be viewed in Settings> Networks tab> Data Manager.
Select the period you wish to view.
This will show you the data usage for the selected period on top, along with app data usage at the bottom.
You can also track your Wi-Fi data usage.
Select the menu overflow while in Data Manager and place a checkmark next to Show Wi-Fi usage.
You will notice an additional tab separating mobile and Wi-Fi data.
Data usage is measure by your phone. Your carrier may account for usage differently.