Custom report: How do I get worst performing services for PLE

I am trying to create a custom report in diagnostic manager to show the worse performing sql server instances for certain counters. I notice that when I add PLE, it shows me the best not worst performing servers since higher PLE is better than lower PLE.  How do I get the custom report to sort correctly?  Is there some way to use a custom metric?  Couldn't figure out how to create that correctly.