Threshold Scope and Reference Axis not available to amend

Threshold Scope and Reference Axis not available to amend

Hi All

I have a report built where the Y Axis is made using a simple formula to return a % result. 

I want to add a target, static line in to the Y axis using threshold. However, when I do this I cannot select Scope of Axis to apply to. 



If i add 0.45 to get 45% it is automatically assigned to the X axis.