Overlay Graph: It overlay the content of two graphs that shares an ordinary x-axis. Left Y-axis on the merged graph show's the present graph's value & Right Y-axis illustrate the value of Y-axis of the graph that was merged. Correlate Graph: Plot the Y-axis of two graphs next to each other. The active graph's Y-axis turns out to be X-axis of merged graph. Y-axis of the graph that was merged becomes merged graph's Y-axis.