JFET's Understanding - Voltage Controlled Resistor
JFET (N-channel) operation in ohmic region is described by the following relation:

\$
I_D = 2 K \left[
 (V_{GS} - V_P) V_{DS} - \dfrac{V_{DS}^2}{2} 
\right]
\qquad\qquad
(0 &lt; V_{DS} &lt; V_{GS} - V_P)
\$

where K is a constant depending on the fabrication process and \$V_P &lt; 0\$ is the pinch-off voltage, i.e. the value of \$V_{GS}\$ for which the drain current becomes virtually zero.

Notice that the behaviour in that zone is not perfectly linear in \$i_{D}\$ vs. \$V_{DS}\$ (because of the quadratic term). If drain-souce voltage is much smaller than the overdrive voltage \$V_{GS} - V_P\$ then you can approximate that relation to a straight line. This accounts for the ohmic behavior.

In this situation the overdrive voltage (a.k.a. simply gate drive) sets the "resistance" of the JFET, i.e. the slope of the straight line. If you keep the drain current constant, changing \$V_{GS}\$ voltage will change the slope and therefore the corresponding \$V_{DS}\$ level.

The graphs you posted are usually too compressed to let you see any appreciable difference in slope in the ohmic region. Some JFET especially built to serve as variable resistors usually include an expanded graph of the ohmic region, i.e. the output characteristics of the JFET "zoomed in" where \$V_{DS}\$ is smallish (~5V max). In these graphs you can appreciate the different slopes.