with a proper graph, on can see what size resistor touse, with a non-linear device.
If one draws a straight line from the supply voltage at 0 current
through the desired current point on the selected diode line,
to the current line, at 0 volts,
you can use that currentdivided into the supply voltage to tell you the desired resistance.This is called a load line( it still uses ohm's law ).
One can draw a new line for each resistor size and seewhere it crosses the LED's line.
So since point C is about 15mA it would be 5/.015 or 333.3?
Sorry, you drew your graph wrong.You have put point B at 13 mA, not 15, and you have not drawn zero volts.
(5-1.5)V/0.15mA = 233Ω
The voltage of his second chart doesn't go to 5 volts.
The larger the voltage drop, the smaller the differencein current is.