Add threshold with value based on current month

Hi there,

I try to add a threshold on my graph and the value need to be based on the current month. Each month the threshold need to change.
I try to create a variable and add it in the threshold field but it only allow numeric value and doesn’t want to accept my variable.

To be honest i’m quite new in the garfana world and i don’t now if i’m in the right way to do that.