TreeMap Visual Problem

[SO]

Help wanted to build a simple TreeMap where the "Group" can be summed.

I have a TotalField that shows the totals for a day.  In other visuals I can insert it into chart and then "Count" or "Sum" or "Average" etc.,  

When I drag my TotalField into the TreeMap "Group" I can see the daily totals, but it will not allow me to "Count" or "Sum" or "Average" etc., etc. .

Any thoughts?