The following shows how to use the two methods. They are basically the same as the two tabs in the Insert trendline dialog.

Trendline = PointSeries.CreateTrendline(Graph.ttPower)
Trendline2 = PointSeries.CreateModelTrendline("$a*x^$b+$c", {"$a": 27, "$b": 0.5})

I can see that the documentation for the two methods is placed under the Graph module instead of TPointSeries in the plugin documentation. I will get that fixed.