On the Mac:
Either in Finder double click Applications/Utilities/X11.app
Or “xterm” in some shell

In another shell do “ocamlmktop -o mytop graphics.cma” 
then “./mytop”.
wait for xterm window to appear
In OCaml REPL do “open Graphics;;”
then “open_graph " 300x100+50-0";;”
then perhaps “set_window_title "Demo";;”
then “resize_window 500 300;;”
then “for i=2 to 100 do for j = 3 to 100 do plot i j done done;;”

Consult Chapter 25 of OCaml Reference Manual.

finally “rm mytop”.