Trend in Filament Costs?
I'm not clear as to the intent of your question, but I would like to provide some insight. Typically, the biggest differentiating factor between a mediocre data scientist, and a good one, is based on the hypothesis they put forth. Therefore, understanding where the cost of filament is derived from is much more important than analyzing the market price equilibrium over a period of time (Even if done with consideration to various filament types).

Here are some basic costs:


Raw material (the cost based on region, and grade)
Manufacturing scale (Mixers, extruders, cooling, spooling, packaging, etc.)
Shipping (Often 50% of cost for small quantities)
Supply chain (Number of middle men)


Without going into detail of every preceding point, I was able to break down costs to a theoretical $10/kg for ABS, if starting with virgin pellets, and shipping flat rate USPS within the US. 

The point that I am trying to make is, fundamentals over technical analysis in this case.