Thanks to everyone who had read my topic.
The interface I used is right, but I made a stupid mistake while I made the call.
Look at the class PerformanceGraph in namespace Statistics. I should have defined the class as following,
[System.Xml.Serialization.SoapTypeAttribute("System.Statistics.PerformanceGraph", "urn:iControl")] //I missed this declaration
public class PerformanceGraph
{
public string graph_name; //The key or name of the graph
public string graph_title; //The title of the graph as appeared in the GUI's Performance Graph
public string graph_description; //The text description of this graph
};