I saw on a screenshot from a Galaxy S2 a dotted line predicting data usage based on current usage which I’d not seen on my Galaxy Nexus.
I had a look and it was true, it wasn’t there, I checked then menu to see if there was a way to turn it on, and stumbled across what seems to be a bug.
Here is how to repeat it, this is on my UK Galaxy Nexus running stock Android 4.0.4.
Go to settings and choose Data Usage
As you see, no prediction line.
Select Show Wi-Fi usage from the menu.
Press the Wi-Fi tab and then go back to Mobile
There you go, the dotted prediction line is visible.