The unit Graph exports functions and procedures for graphical output. It requires at least a VESA compatible VGA-Card or a VGA-Card with software-driver (min. 512Kb video memory).
Before the graph unit can be used, be sure your graphics adapter supports the VESA-Standard. Otherwise in the most cases you can try to use a VESA-TSR to make your adapter VESA compatible (e.g. UNIVBE).