The following circuit is used to plot the power vs the load resistance curve.

The PSPICE script is as follows.

MPTT grapgh
V1 1 0 DC 10
Rt 1 2 20
Vx 2 3 DC 0
RL 3 0 RMOD 1
.MODEL RMOD RES(R=1)
.STEP RES RMOD(R) 1, 100, 1
.DC V1 20 20 1
.PRINT DC V(1,2) I(RL)
.probe
.end

The resultant plot as a function of RL is shown below.