Quick update: The first try using ABM blocks resulted in the same error as attached previously. In troubleshooting the error, I realized that the first method of defining Vgs-Vt explicitly in the "Trace Expression" field worked just fine. The error is due to the fact I'm running a parametric sweep so Vgs-Vt is actually a family of curves. Essentially the x-axis can't be a family of curves. If I turn off the parametric sweep then both methods work just fine; I get the plot that I expect. Since I need to run parametric sweeps to have useful gm/id curves, it looks like exporting the data and using external tools might be the only way to do this.