A coordinate plane formed by a first coordinate axis associated with a parameter (A) and a second coordinate axis associated with a parameter (B) is displayed. A first reference line indicates a first reference value (A1) of the parameter (A). A second reference line indicates a second reference value (A2) of the parameter (A). A third reference line indicates a first reference value (B1) of the parameter (B). A fourth reference line indicates a second reference value (B2) of the parameter (B). A first plot indicates a combination of values of the parameters (A) and (B) that are measured at a present time point. Each of second plots indicates a combination of values of the parameter (A) and the parameter (B) that are measured at one of past time points. A region defined inside the reference lines is displayed on such a position that includes a center of the coordinate plane.
