Overlay Graph: overlays the content of 2 graph tat shares a 
comman X axis,left Y axis on the merged graph shows the 
current graphs value n right Y axis shows the value of the 
Y axis graph that was merged.

Corelated Graph: corelated graph plots the Y axis of 2 
graphs agains each other.the active graph Y axis becomes X 
axis of the merged graph,y axis of the graph that was 
merged becomes merged graphs Y axis.