I have this issue now.
I need a solution too.
The guy who had it working, he only had lines and they were step lines.
When I set for step lines and bullet set to circle, I get the tooltip.
Bullets set to circle isn't going to work for me though. I still need a solution.
I am on 8.1.19.
@milbar How did you make tooltips on your line renders on your xy chart?
I have many issues with XY Chart right now.
Xy chart help, named query, settings changing]