I was curious about your chart and I test some of my IRF540N.
I used three IRF540N plus one IRF630 and one IRF640.
A 12V power supply was used with a 12V 2A lamp. This explains why the maximum current is 2A.
The ambiant temperature is around 20 celsius. Each Mosfet was attached to a huge heatsink to keep the TO-220 case at the ambiant temperature.
No heatsink on the mosfet and it heats up then your measurements are all wrong. Heat reduce the Vgs.
On 1 experiment I put the IRF540 mosfet #2 at near 0 celsius by putting the heatsink into a bath of snow. I just grab some snow from outside. I'm in Canada
All my mosfets need at least 3.2V minimum to get ~10 ma on the drain. The IRF540 #2 start to get 10 ma at Vgs=3.6V when the heatsink was near zero celsius.

- Vgs VS Ids
- mosfetVgs_vs_IDS.png (84.85 KiB) Viewed 1135 times
My chart is different than yours. Temperature, single mosfet measurements and variability between parts . This is why Vgs bellow 4 V is not there.
I'm sure all the diagrams in the official specifications are worst case scenario.