How do I add a trendline

I have 2 rows of data, both plotted into a 2d stacked column chart.

I want to have a trendline appear on the chart, the manual says to select a series of data, but the data I want trended is already plotted and "trendline" is greyed out in the inspector.

So, do trendlines only work with additional data? ie would I have to add a 3rd row of data to calculate the trendline?