I like the app, but it would be better if it had a widget feature and could sit on one of the pages and give the current usage info instead of having to login to see it, like that app from Alexandre Jasmin.
I'm on Android, btw.
Interesting idea but the app would need to connect frequently to keep you updated, which would make you use more data. Why not search in the playstore, I remember using data counter widget few years ago and it takes the info from the phone so need to use data to see your data usage.
It would be better if it actually maybe 4 times a day updated the data counter on your phone to show your estimated usage