Does anyone know examples of real-life data sets {(x_1,y_1), ... ,(x_n,y_n)} that are monotonic in the sense that either (i) for any two indices i and j the inequality x_i<x_j implies y_i<=y_j or (ii) for any two indices i and j the inequality x_i<x_j implies y_i>=y_j? Here, the x_i's and y_i's are real numbers.