name = "non_oriented graph";
directed = 0;
nb_summits = 3;
tail = [ 1 2 3 1 ];
head = [ 2 3 1 3 ];

g = make_graph ( name , directed , nb_summits , tail , head );

g ('node_x') = [ 100 100 500 ];
g ('node_y') = [ 200 500 300 ];

show_graph (g);
show_arcs([1 3]);