Is this way of plotting AC current gain in LTspice correct?
A better way would be to insert a voltage or current source with an AC source and use an AC simulation. This is an example (unrelated to your circuit) below:



Above I have plotted the current gain of the circuit, the circuit needs an AC source to run an AC analysis instead of running a transient analysis. One could find the AC current gain with a transient analysis but only at one frequency (unless they ran a source with a frequency sweep) 

Use the small section of the voltage parameters to set the AC amplitude of the source used in simulations.