I deleted several legend entries, but now I would like to bring them back.
How is this accomplished?

Here is the purpose for deleting them in the first place. Maybe you can
help me with an alternative. I have a list of numbers that I am graphing. I
insert a trendline to get a smoother line. I also insert a single point to
show the point of interest. I format the original line to not show (set Line
and Marker to none). I then delete the legend entries for the trendline and
orignal data set. This way the legend is not cluttered. I wanted to tweak
it a little but I can't access the orignal data curve, as it is not showing
in either the graph or the legend.