This might sound counter-intuitive, but the graph will show what was left at the end of the day, not what it starts with. Therefore, looking at March, it looks like 10-6.3GB (or 3.7GB) was used in the first day. Judging from the slope of the curve, that usage seems consistent and would definitely run out by the third day. I would seriously look for what might be draining that fast, which is typically something like movies or TV shows that are recording on-demand rather than live. Doing a lot of 'cloud' operations such as Google Docs (et. al.) will also use a lot of data, and might contribute to that.
... View more