Can I account for Mobile Data and WiFi data separately?

A

AC Question

I've set up my wonderful Nexus 6 pretty much as i want it but have one outstanding problem, checking Mobile Data from my contract allowance of 500MB, I cannot see how to differentiate between WiFi Data and Contracted Network Data which i understood to be MOBILE DATA, The warning message of the cap is coming up after 500MB of data INCLUDING WiFi data despite me specifying the cap as Mobile Data. What am I doing wrong? Help PLEASE :)

It would be nice to see BOTH reported SEPARATELY but the NEED is to limit Mobile Network usage so as to avoid excess charges.