Need to create a report where columns denote months, rows denote completed tasks across month.

I need to create a report such that:
The columns denote the months (Jan Feb ... Dec)
The rows denote the no of completed tasks across instances in that month per User.

Something like this:
-----------------Jan---Feb---...---Dec---Total tasks completed
User1--------10-------20---------10------40
User2 ---------05--------0----------0-------05

The tasks are aggregation of Completed tasks across process instances for that user.
How can I achieve this ?....

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