JFET - transconductance, gain(mu) and internal resistance at the drain
MU at 6 volts drain is just [Id(Vgs = 0.6v) - Id(Vgs = 0.2v)] / (0.6v - 0.2v), or

(13.5ma - 6.5ma) / 0.4v =  7ma/0.4 = 7ma*2.5 = 17.5mA/volt