I've got a chart using two data series plotted on separate chart axis. I
need to show a trendline for each of the two data series. If I chart them on
the same axis, then the trendlines show up as expected. But I really need to
have the second data series plotted on a secondary axis. When I move it to
the secondary axis, the trendline disappears. It's still logged in the
legend, but just doesn't show up.

How do I get it to show up?