View recent records for a cumulative sum over all records

I have created a chart that shows the cumulative sum of a field month on month over all time. I would like to place this chart on a dashboard and only display the all time cumulative sum for that last 6 months. Could anyone tell me how I can achieve this?

Currently when I place a time filter for the last 6 months, it begins summing from 6 months ago.


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