Insert brackets, based on how the graph looks

If you take a look at the attached file, then you see that it makes a bracket between the red and the green graph.
The problem is if there are any other numbers in the graph, it will continue to draw the bracket in the same place.
What I want is that the top of the the bracket should be on the red line and the bottom of the the bracket should be on the green. Then the the bracket will be right, regardless of the numbers that are entered.test.xlsm