Add horizontal line to a chart

[Daniel]

How can I add a horizontal line for my chart?

I did choose the XY value, paste to my chart but what I see is just a dash ?
I should have a line from x min to x max ?

Any ideas of this? Do I have to add trend line?

Thanks
Daniel

 

[Jon Peltier]

Are the X values of the added line consistent with those of the original
data? What is the original data like, and what is your added series' data
like?

- Jon