Yes, one has to check
-if the graph has points in increasing order in all cases (if not -> sort
into a temp graph)
-if the request is a a closed graph (main use is to compute the integral
inside a TCutG). This is a bit more difficult.
The user could instruct integral about the above cases or specify the
type of integral that he/she requires.
If somebody is willing to contribute, let me know.
Rene