Detection of repeating values over a period

I have a log that contains details of policy violations committed by users and this is available for a period of few months. I would like to find users who are repeatedly committing the violations over a period of time.

If I do "time chart timechart limit=10 span=1w count by User_Name useother=f usenull=f", I only get top counts of users over time and not exactly repeat offenders.

Any pointers in this regard would be great.

Thanks