The filter statistics portlets will currently only allow you to choose fields that have clear, defined boundaries, like select lists or radio buttons. Grouping by date field i.e month or week would be ideal but has not been implemented in JIRA yet so it this makes creating this portlet difficult.

I will change this to an improvement but I cannot specify a version when this will become available.

Thanks,
Brian

Thanks for your answer, we'll be happy to see this feature in the future, despite what version will fix it )

Cheers,

Andrés.