Subtract the current row with the previous row

Hi I have a below requirement.

I have a table named forcast table with forecast version and values.

I need to initially calculate the sum of values for each forecast. and subtract the forecast from the previous row(for 2022.7: I need to take 2022.8 and subtract 2022.7, for 2022.8: I need to take 2022.9 and subtract 2022.8 and so on)

Atlast, I have to build a graph with the differences. Please find the clear requirement in below link