Overlay Graph: It overlay the content of two graphs that shares a common x-axis. Left Y-axis on the merged graph show.s the current graph.s value & Right Y-axis show the value of Y-axis of the graph that was merged. Correlate Graph: Plot the Y-axis of two graphs against each other. The active graph.s Y-axis becomes X-axis of merged graph. Y-axis of the graph that was merged becomes merged graph.s Y-axis.