Hi Johan,
Greetings from Syncfusion.
A trendline is a line superimposed on a chart that shows the data's overall direction. Syncfusion chart supports 6 types of trendlines such as linear, exponential, power, logarithmic, polynomial and moving average. We have an online demonstration where you can see the differences between each type. In source level, we have used formula for calculating each trendlines.
Please revert us if you have any concerns.
Regards,
Durga G