help in using ADS tool in LNA simulation

I want to create a plot of NFmin vs Vgs in ADS 2009........But by default the x axis is having frequency as the independent variable.....how do i make Vgs as the independent variable?............

Assign a single frequency in s-parameter simulation block and then assign your Vgs as a variable in the same block, it will work.

Thnks a lot....tht worked. But, i couldnt set the variable Vgs by double clicking the S parameter simulation controller and going into it.....i had to set the variable by enabling the sweepvariable display in the schematic and then setting the sweep variable from the schematic display.....isnt there a way where i can set the variable Vgs by double clicking the S parameter simulation controller and editing the settings in that window?.........