The MRTG graph will probably only show what you were using, not the capacity of the circuit.
If it hits the 2Mb level, then you got 2Mb at that time. Try one of the online speed testers, like speedtest.net, when no-one else is using it, which should give you an idea if the line is truly 2Mb. It also depends on how the router is configured, as to whether you really have 2Mb or there are some signalling as well that takes out a 64kbps slot. So it could be a bit less, legitimately.
If you have a Cisco router on the end, that can tell you how loaded the line is, and the speed it is set to be. That may also give an indication.
Just realized you posted the graph below. I would say from that graph, that you are getting 2Mb/s. You are unlikely to generate it all the time, but you can see that it hits this level quite a bit, so it looks OK from that picture.
Sometimes you just have to trust the ISP!