A displaying chart image in a bitmap format is generated from ledger sheet data, while identifying layout information capable of identifying a plurality of line drawings on the chart image respectively is generated for a mouse decision. Then, identifying information corresponding to a dot designated through a mouse cursor over the displaying chart image is acquired from the identifying layout information and a dot string indicated by the identifying information is specified as the line drawing. Consequently, it is possible to increase a display speed more greatly as compared with the case in which the line drawings are subjected to vector rendering and are thus displayed respectively, and to identify the line drawings in the chart image formed into a bitmap based on the identifying layout information, respectively.