Originally Posted by djmcnz
It is on/off also.
Uncheck the box that says "Set mobile data limit"...
EDIT: Figured it out.
The checkbox will enable/disable the hard limit - where if the limit is reached, your data will be shut off until the next cycle (cycle is selected by you in the same screen)
The graph shows two limit bars - orange and red. Orange is the warning level and Red is the limit. It's not intuitive at all, but you can grab the right side of the bar and drag both the red and orange bars to whatever level you want. When your usage crosses the orange threshold, a warning appears that you cannot dismiss. When it crosses the red, I suspect you will get another harsher warning (or if the checkbox is checked, your data will be disabled).
This looks like a very nice system for people on the non-unlimited plans. I'd prefer it if the warnings could be dismissed in some way - since once you hit the warning level, your notification bar will have that warning in it for the rest of the MONTH! It would also be much nicer if you could specify a number instead of awkwardly dragging a line. I'm not OCD, but I still had to spend a long time dragging up and down to make the limits be round numbers!