The post aggregate filters work only with the include and exclude functions. Numerical functions can not be applied to these. There might be another way of doing what you want.
You can get the volume of events per minute using the sum function. Your search would look something like this:
| _minute.count() _type.equals(my-data-type)
It doesn't quite get you as far as you would like. We're currently investigating a set of new analytic functions that will give you min/max/avg for your use case. Could you update the poll with any analytics/functions that you would like included in a future point release here: logscape.activeboard.com/t59107359/poll-for-new-analytic-functions/