A line graph of what exactly?
Can you attach a Tableau Packaged Workbook with some sample data?
I have included a workbook. In this workbook I don't have any formulas or the sort. I had to use Excel's random generator as sending even real dates would break our proprietary and confidential internal regulations here.
The line graph would depict volume at any given time, the volume would be the problems that are currently open. That are not closed at that point in time.
The FAQ: Open & Close Date link above references Showing Records That Fall Within a Period of Time | Tableau Software which should get you what you need after a small modification to change into a line graph at the end. In the same thread, I posted a link to a workbook where I was able to apply this successfully.