Light Bulb Experiment: DC Voltage?

In continuation of my previous post on this topic, I have set up my lightbulb with a 2.2 V DC voltage for 10 minutes and measured the resulting output voltage and current:

Over time, the current declines somewhat--exponentially, in fact--whereas the voltage measured across the bulb remains ~constant. The implication here is that the resistance of the bulb slowly increases with time, presumably until it reaches some quasi-steady-state temperature (and hence resistance).

The implication for this is that the "linear" regimes of the lightbulb's voltage-current curves aren't actually entirely linear. That said, they appear to be reasonably close to linear over the short-term (~2-10 seconds) considered in the ramps yesterday, at least as observed using a DC signal. In fact, I will close by posting a set of four more images, to compare the upper/lower linear regions for short time ramps to longer time ramps.
Here is the upper linear region linear fit with a 0.002 Hz frequency (500 s period):

And here is the lower linear region fit, for the same data:

Now let's look at a 0.2 Hz signal (5 s period), beginning again with the upper linear region:

And the lower linear region:

That's it for today. Next week, God willing, I would like to start looking at the intensity output for the light bulb. Preview question: if we treat the resistance as constant when the bulb is "on", we should expect to get what type of graph for intensity vs voltage and intensity vs current?