srishino:
The ‘fill’ option only ‘closes’ the graph between its first and last points (with a virtual straight line between them), and then just fills the resulting enclosed area with the fill colour.
To see this, run couet’s ‘twographs’ example of Jun’07 as it is and it looks ok, but now change the the fill of gr2, (e.g. gr2->SetFillColor(1); ) and you’ll see what is actually happening. His example works because of the shapes he used but your case is different.
So, use whe method shown by Rene Brun (grshade.C), which is the same idea proposed by Wile E, and also here:
Make a band with two lines
The idea is explained in my post in that link: order the 2 original graphs in increasing x and then use them to fill a new graph, so that one graph (with n1 points) goes to points 0 to (n1-1) in the new graph and the other graph (n2 points), is filled in reverse, i.e. goes from point (n1+n2-1) to n1 in the new one.
Then draw the new graph with the fill option.
-Daniel