Plot Stepped Measured Data is disabled at Spice Error Log LTspice
Not only your .MEAS is wrong, but I'm used to a different syntax, but you are using undeclared values (n? did you mean t?). Try not to rely on the UI too much and, instead, use the manual, or ltwiki. You would have found out that the proper expression for AVG measurements is (as given in the manual):


  .MEAS TRAN res7 AVG V(NS01)
  + TRIG V(NS05) VAL=1.5 TD=1.1u FALL=1
  + TARG V(NS03) VAL=1.5 TD=1.1u FALL=1


Because your directive could not be executed, there were no measured values, and so there's nothing that can be plotted. If you change your .MEAS card to this:

.meas x avg v(x) trig time=t-S/2 targ time=t+S/2

you'll find that it works. Not home right now, but here's the proof: