LTSpice sweep resistance
Here is an example how you can calculate the power dynamically:
.step param Rload 1 10 1
.meas VRMS RMS V(X)
.meas VAVG AVG  V(X)
.meas PRMS param VRMS**2/Rload
.meas PAVG param VAVG**2/Rload


Run the simulation



Open the Spice error log (ctrl + L)
Right click and select plot steped meas data
Plot the power that was measured against the resistance as shown below: