A vector data selection unit selects, from a vector file of a past line graph, each vector data of a time period where a new line graph overlaps. An attribute value setting unit sets, of the vector data selected, a high transparency degree value to oldest vector data, sets a low transparency degree value to newest vector data, and sets an intermediate transparency degree value to intermediate vector data. A graph display unit displays a portion of a time period of the past line graph where the new line graph overlaps, with a gradation.