We demonstrate this with a simple coupled line structure as shown below.   This is an EM layout in AXIEM with 25mil wide lines, 250 mils long on a 32mil dielectric with Er=3.38.   

When this structure is simulated and s-parameters are plotted, the results will look like below. 

Next, we need to parameterize the EM structure.    We select both shapes and select Draw > Parameterized Modifiers > Spacing Modifier.  Then draw the spacing modifier between the two lines at ports 1 and 2.  The result is shown below. 

Now we need to sweep the spacing.  First set the SP parameter of the shape modifier to a variable and draw the variable on the em layout using the Draw > Add Equation. 

Next, right click on the S=15 variable and select Setup Sweep.  From the dialog that opens, enter the values below and click OK. 

Note: when you parameterize an EM structure, right click on the EM structure and select Preview Geometry to see what the structure layout at each sweep point.  

Sweep point 1:

Sweep point 4

After simulation, the graph will show the results from each sweep points. 

Next, the EM is added to a circuit schematic as a subcircuit.  The easiest way is to use the Ctrl+k hotkey and choose the EM simulated before.  Then add schematic ports to the subcircuit.  The example below also assigned the M2CLIN symbol to make things easier to wire together. 

Simulating this schematic will show the results with a spacing of 16 mils. 

By default, the parameter S of the EM subcircuit will have discrete values used in the simulation. 

You can change an interpolation option on the EM document to allow the parameter to interpolate between the values.   Right-click on the EM structure in the project tree and select Options.   Find the Interpolation/Passivity tab and check the Enable parameter interpolation checkbox. 

After this change, the S parameter in the schematic can be entered as a continuous value. 

Note: interpolation up to 10% of the parameter value is allowed until a simulation error will occur.