"Trend Lines" greyed Out

I have a simple data set of approximately 100 pairs of floating point values. The Calc program reads these data and plots it as an X-Y scatter plot correctly.

I want to test some different functions and get the idea from the Help that this is possible. However, when I try to insert a Trend line, from the insert menu that feature is greyed out.

I am using Windows 8.1 with LibreOffice 4.2.5.2 I am probably misintrepting the help information on this feature, so is there some step that is implied but not explicit?

Thanks for any help.

Select the plotted data series first in the diagramm by clicking on it. You then can also go via the context menue.

Lupp,
Thank you for your answer. I had been just double clicking on the background of the plot. When I double clicked on the data line on the plot, Trend Lines were no longer grey out.

None of the canned expression fit my data well and I will try to learn to use the context menu with an expression of my choosing.

Thanks! Someone should correct Trend Lines - LibreOffice Help but I don’t know how to make/propose edits https://wiki.documentfoundation.org/Talk:Development/Wikihelp#Editability