I have a scatter chart with sparce data.  So to make the lines more
clear, I have added trend lines.  I did this a few weeks ago and I got
the whole thing formatted the way I want.  Now I have added a new data
series to the chart and when I tried to add a trendline it shows up in
the legend.  But the data series is already there!  So now I have four
data series with lines in the legend, along with the new data series as
well as the trendline separately.

I thought I might be able to fix this in the "Format data series"
dialog box, but when the trendline is in place, I can't select this
popup menu.

What am I doing wrong that I can't get the legend to display just the
data series and not the trendline?