I need to add difference for Average Row at the end

I need to add difference for Average Row at the end


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Guys, hello! 

I have this graph, it calculates the averages of Retention and then shows the delta (2024 - 2023). In the 3rd column with delta Churn %, I want to show the difference between averages too, which would be (16.99 - 18.66 = -1.67). 

How can I do it?