Calculate change of running total current year compared to previous year

Hello to all pivot table experts. I have a table with per month sales data dating back for three years. I have already figured out how to calculate the cumulative sales data by using "show as - running total in - month". What I would like to do now is to show the year to date percentage development compared to the same period of the previous year. Using "% running total in - month" ony compares the individual months, i.e. March 2019 to March 2018. What I'd like to have is a column showing January to March 2019 vs. January to March 2018, comparing the cumulative values with each other. I hope my explanation is clear, sorry for the clunky English...