At any constant AC voltage, the slope of the I-V plot is the filament's conductivity at constant temperature (as the Joule heating is
I squared R, there are 120 heating cycles per second and insufficient time for the filament to change temperature).

Notice that the slope is a straight line, indicating that Ohm's law is obeyed. Now notice how the slope changes when the AC voltage is
increased or decreased, showing that the filament's resistance changes when the filament temperature changes.
Hence, we see that Ohm's law is valid at a fixed filament temperature, but fails when the filament temperature changes.