Constant lines by a mouse click?

[User (Legacy)]

Probably a very simple problem, but as a new registered user, I want to 

place a constant line in the X axis by clicking in the chart window at the 

appropriate point. I then need to know the exact X value I have selected so 

I can pass this to another form containing another chart to display the 

'same' constant line on that chart too. In a way, I can then sync these two 

charts points for the user. 

How do I do this - a gentle shove in the right direction would help greatly! 

Neil Hall