Well looking at that graph the internal 'pull-up' look pretty linear to me. I think what you want to determine is what is the output impedance of a digital input pin when it's internal pull-up is enabled. So looking at the graph I would look at the current value when the pin is at 1/2 Vcc or 2.5vdc and it would seem that it is about 70ua. So R= 2.5/70ua = 35.7 kohms, which should represent the output impedance rating for the pin.
As the spec is nominal and subject to tolerances, you might want to run an actual test on your specific pin. Wire a say 100k pot, one end and the wiper from pin to ground and decrease resistance until pin voltage is equal to one half Vcc, then remove the pot and measure it's resistance and that should be equal to the output impedance of that input pin when the intenal pull-up is enabled.
Lefty