There is a TOP users portion of the xFi information screen and a Devices list (though you have to drill into each device to see its usage in percent).
Now that I'm paying extra for data overages it is time to DEMAND that we get those percentages listed in GB, since that is what we are being charged for. 50% of data usage on a 1GB day is useless information, since that device would only have consumed 500MB for the day. 50% usage on a 50GB day being 25GB would certainly get attention. Though we have NO way of knowing what 50% use day is which, making it near impossible to catch the offending app/website that is continually streaming data in the background.
I will call and refused to pay the overage charges until this feature is implemented, and I urge ALL of the other xFi customers to do the same.