This is because the results of an AC simulation are complex numbers - you get a waveform of these complex values versus frequency. Because they are complex, it allows you to find the mag, phase, real, or imag of the waveform. When you plot VF() without any modifiers, it plots the magnitude automatically.

However, the xmin, ymin functions etc do not make the same assumption. Instead they don't know which minimum you are talking about - and just return nil. So if you want the x value for the minimum magnitude, do:

xmin(mag(VF("/I0/PLUS" )) 1 )

Regards,

Andrew.