Average days to close requests

Average days to close requests

Hi,
 
I need a report which just says how many days, on average, it took to close requests, by month, for the previous 12 months. I would also like to see the result in a graph.
 
I'm using version 7002 at the moment. We don't enter time on tickets, so I'd say we should would use the created time and completed time stamps.
 
Is this possible?
 
Thanks
julian.

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