Mapping two characteristic curves of a power amplifier to each other
The lower curve is the power product of an AC coupled signal.  The Psat is the maximum power due the knee or saturation limit defined by a distorted peak of the entire sine waves limited by the "knee" of the upper curve on the left and clipped by Vdd on the right.

Point A is out of reach at max current because there are resistors in the path Rd,Rs to achieve a gain and limit current.

Point B is beyond the sine wave if the input were increased to cutoff current then Vds=Vdd when the JFET resistance is high.

Imagine a small signal input on the top curve (around Q on the load line)  This is a just an RMS power point near the origin on the lower linear power curve.

Note: On chart 1 when Vds=2.1V, Id=3.9mA when Vgs = -0.1V peak swing. This implies Rd+Rs = (12V-2.1) / 3.9mA= 2.54 K  then Vds/Id=Ron

Now can you figure out gm the inverse or ΔI/ΔV=gm from Vds and Id?