TGraph::InsertPointBefore: Tail insertion
TGraph::InsertPointBefore: Tail insertion

When user invoke `InsertPointBefore()` with an empty TGraph (fNpoints==0), they will receive a message:
> Error in <TGraph::TGraph>: Inserted point index should be <= -1
This is confusing. The point index can not be -1. And it prevents insertion to an empty graph. Also, people who familiar with `std::vector` may expect the behavior likes `vec.insert(vec.end(), value)` to be effective.
olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root