Trendline that works with categorical values wanted!!
With the march/april 2017 update you can now incorporate a trendline. However it only works with continuous variables on both axes on charts and trends is almost always about development along time (by definition categorical value). You can and could for years incorporate a categorical value (time) and make a Trendline in Excel. How about that you get this to work in Power BI? Right now Trendlines is pretty much useless, unless you completely want to rewrite time dimension to values (and you do not want that).