Add SUM function for graphs

I may just be missing it, but I'm not seeing a way to display the sum of the history of an item, nor a feature request for it.  I understand this may be accomplished via calculated items, but calculated items doesn't provide the flexibility of selecting an ad-hoc time period to sum.  This function would be useful in conjunction with the pie graph, or even a new "Text" graph that simply displays the item value and unit.