Hello Tobias,
Yes, the trendlines are not supported at this moment but they will be added in V3 release( likely with the source code as example of extending chart possibilities). I will try to post the preview source code here during a couple of weeks.
--Best regards,Alex
Attached is preview of C1.WPF.C1Chart.Extended library which includes plotting of trend lines and functions . The zip contains source code of the library and sample project. The library will be included into V3 release.