How to link X-axis / Y-axis constant line to a column/function rather than Hard code it?

You can see below a simple scatter plot and I want to add axis constant lines. But instead of hard coding it, I want it to be linekd to the input box. 

So, what ever number the user times in the box below the constant line should reflect it.

I heard that there should be "fx" button nexct to Value input box (bottom right) but I cannot see it.

Any advice/help

Thanks