If you are using timechart you can use span option to do it.
Description: Sets the size of each bucket, using a span length based on time or log-based span.
Alright, I don’t know if there are any xml option (simple or advance) to do it but you can bucket your data in the time interval that you want and use it in the xseries of your chart in spite of using the default timechart capabilities.
I mean, | bucket span=6m time | eval mytime = strftime(time, "%H:%M:%S") | stats xxx by mytime
This should force the chart to mark your frequency.