I know how to plot a graph in google spreadsheet. But how to plot multiple graphs, with different X and Y values in Google Spreadsheet?
By multiple graphs do you mean multiple axes on the same graph? If so this is not the answer you were hoping for, but it currently can't be done :( Hopefully they will add this feature soon.
You can plot multiple axes using the Google Chart API, but that probably isn't useful to you.
plot multiple graphs, with different X and Y values can be achieved in New Google Sheets:
but with what appears to be very limited scope for tailoring the result to suit (relative even to Excel).