After the simulation completes and the DC results are plotted, do the following:
1. Open the Calculator and create an expression for the x-axis quantity. E.g. VS("/gate") - VS("/vdd").
2. Make sure the plot mode is set to "append" and plot the expression. The expression should now visible in the graph window.
3. Double-click on the x-axis to access the axis attributes. Change the "Plot vs." field to above expression and "OK" the form.