In the analysis what do you understand by Correlated graphs and Overlay graphs?

Both the graphs result in merging of two graphs and showing the results in one graph. But there is a main difference between them. In overlay graphs: The X-axis will be shared by both the graphs. I.e X-axis will be same for both. Where as left hand side Y-axis shows the current graphs results, and right hand side Y-axis shows the results of the graphs that is merged.
In Correlated graphs, Only Y-axis of both the graphs will be compared with each other. The current graphs Y-axis will become the X-axis of the current graph. And the merged graphs Y-axis will become the Y-axis of the current graph.
Explain vuser_init() function?
Vuser_init() contains the statements related to logon to the server. It contains the steps to be performed only once at the beginning, out of all iterations of the script execution.
Explain vuser_end() function?
Vuser_end() contains the statements related to logoff from the server. It contains the steps to be performed only once at the end, out of all iterations of the script execution.