let xinit_coord (nb_agents : int) : unit =
xmax := (min (float_of_int(nb_agents-1)) 9.) *. 10. +. 5.;
xscale := (!width /. float_of_int(abs(int_of_float(!xmax -. !xmin))));
nbAgents:=nb_agents;
Canvas.configure canvas ~scrollregion:((xconv ((-1.) *. 10. -. 5. )) - (5*(get_max_length_message()) ),0,(xconv ((float_of_int(!nbAgents + 1)) *. 10. +. 5. )) + (5*(get_max_length_message())), (yconv !yOrd))