Using an N-channel JFET as a VCR
There exists a polynomial with the datasheet mapped into a spreadsheet that can match the JFET Vgs vs Id such that the incremental resistance can be computed and made smooth enough to demonstrate the Ron vs Vgs shown below.

When one is careful using Irfanview free for Windows,  a rectangle with pixel counts may be measured then one can compute with estimate with more accuracy may be redrawn every 0.25V for Vgs and estimated Id with a few % then compute the incremental ΔV/ΔI= Ron [Ω] then plotted with smoothing skills, one can duplicate the original graph and display Ron.
Forgive me for previous erroneous data.
Keep in mind the tolerances on Idss and Vcutoff are not reflected here.
This is not my answer , just more data for you to create your formula to match the data.
If I make up a formula, it looks like this in yellow

k is related to the chip size and power rating, but I just guessed.