I have used a filter and the parameter is yielding the values.Please find the workbook attached.
Not sure if this is what you are looking in for.
O&C Shared.twbx 650.7 KB
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I've had some success with this, but I'm using Tableau 9.3 so you may not be able to open the workbook I saved.
Since the data sources are small, I made a duplicate data source. In the primary data source, I made a calculated field just for the Close Month (datetrunc('month', [Close Time])). In the secondary data source, I made a similar calculated field for the Open Month.
I made a relationship between the two data sources, requiring that Close Month be equal to Open Month.
In the primary data source, I needed to create a calculated field called "Open Incidents" that referenced the "Date Selector-Open" calculated field in the secondary data source. Then I could add the new "Open Incidents" field to the line graph, and all appears to be working. It matches the Excel crosstab that I made to check the data, and it looks like it matches the first chart that you made.
Let me know what you think.
Your solution is better than mine!