add_edge - adds an edge or an arc between two nodes
add_edge returns the graph g1 with a new edge from node number i to node number j. If the graph is directed, the edge is an arc. The number of edges plus 1 is taken as the name of the new edge.
ta=[1 1 2 2 2 3 4 5 5 7 8 8 9 10 10 10 11 12 13 13 13 14 15 16 16 17 17];
he=[2 10 3 5 7 4 2 4 6 8 6 9 7 7 11 15 12 13 9 10 14 11 16 1 17 14 15];
g=make_graph('foo',1,17,ta,he);
g('node_x')=[283 163 63 57 164 164 273 271 339 384 504 513 439 623 631 757 642];
g('node_y')=[59 133 223 318 227 319 221 324 432 141 209 319 428 443 187 151 301];
show_graph(g);
g=add_edge(1,7,g);
g('edge_color')=[ones(ta) 11];
show_graph(g);