Rosenbaum developed the sliding-square plot to display simultaneously a scatterplot of paired data, a boxplot of their differences, and a boxplot of each marginal distribution. We extend the sliding-square plot to include the two-period, two-treatment crossover sum-difference plot, with corresponding boxplots of scaled sums and differences for each sequence. The plot is useful for displaying characteristics of the data together with evaluating carryover effects, treatment effects and the assumptions under which they are tested.