A graph image generation unit 42 constructs a two-dimensional space and generates data of a graph image in the space, in which a graph G of a type is rendered. A coordinate receiving unit 43 receives an arbitrary coordinate in an arbitrary timeline TL designated by a user operation, among coordinates in at least two axes defining the space. A line image generation unit 44 acquires information of the space from the graph image generation unit 42, and generates data of the line image in the space, the line image passing through at least the coordinate in the received timeline TL. An image synthesis unit 45 generates data of the synthesized image by combining the data of the graph image with the data of the line image 111b in which the line is rendered. A display control unit 46 controls a display unit 17 to display the synthesized image.