If there are multiple statistics, the Overlay option is checked by default, and the Y variables (statistics) are plotted on the same chart for each level of the grouping variable. However, the levels of the grouping variable cannot be overlaid into the same plot frame. For example, if the levels of your grouping variable are Male and Female, the Overlay option cannot be used to combine the two Male and Female graphs into one graph. To see that type of result, use Categories, X, Levels instead of Grouping variables.