can someone explian the metaphor in the following line: "The last sound of the american plane dissipated in the wind and there we were, in our cemetery."

Although I would not call this entirely metaphorical, I believe what you are looking for is that the American plane is a symbol of hope for the people, and indeed the plane has gone and the hope that the people had for their freedom has also disappeared. The cemetery is metaphorical in the sense that they believe that the camp they are in will be the place where they die; thus, when all hope is lost, they remain in a place where they are, at best, among the living dead.