1. A graphic display apparatus, comprising: a display unit; a reference object length storage unit in which specified parts of reference objects and actual lengths of the specified parts are stored in such manner that the specified parts and the actual lengths of the specified parts are associated to each other; an image storage unit in which a plurality of images are stored; an image display control unit which displays on the display unit, an image from among the plurality of images stored in the image storage unit; a size detecting unit which detects an image length of a specified part of a reference object in the displayed image; an actual length setting unit which sets a display range of a coordinate system defined by coordinate axes based on the image length of the specified part and an actual length of the specified part so that a specified image part in the image is a coordinate range in the actual length of the specified part; a graphic display control unit which superimposes and displays a graphic on the specified image based on a user's operation; and an actual length value display control unit which displays a coordinate value in actual length of a point on the graphic based on a user's operation.