'Excel Pivot Chart: Open Closed and Running

I have a database with records that contain a open and closed date, which I want to present in a chart by month so show how many entries are opened per month.

I was able to present both datasets per date according to this approach: https://answers.microsoft.com/en-us/msoffice/forum/all/excel-pivot-table-question/c2796e5b-14c5-4331-bbeb-4058b048148a (Solution according to Andreas Killer)

The results looks like this: Chart

What I want to add is now a running count, that shows the count of created minus closed in the background, therefore cumulating when more records are opened than closed, but I am getting stuck and do not know which formula to use and how to apply this to the pivot.



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