A method for plotting a distribution area of a plurality of data points each having two paired variables in a scatter diagram includes (a) dividing a distribution of data points into at least two division areas in one or more radial directions of the distribution of data points from an arbitrary first central dividing point and selecting a data point having a longest distance from the first central dividing point in each of the division areas as a representative point of the distribution of data points, and (b) plotting a distribution area representing line by sequentially connecting the selected representative points in respective division areas.