Hi guys @sm_mk, @Guido_Stark and @hari! 
Just wanna let you guys know that we have put together a guide for Trendlines in chart here:
<Chart with trend line | Custom charts Library - GitHub>
P.S: we’ve changed this template to a full code example of Scatterplot with Trendline, hoping this would be easier for you to adopt & customize further. Please see here:
<Scatterplot with Trendline Tutorial | Custom charts Library - GitHub>
Plz do check it out and let me know if it’s useful for you 