User Defined Aggregation function
Aggregation function (e.g. Average, Count, Min, Max, ...) seems to be first-class citizen function in Excel.
Only those functions can be used to summarize field values in Pivot Table and in Subtotal function.
As far as I know, I can expose user defined function into Excel through an Add-In but I cannot extend aggregation functions.
Please provide an API to register our own aggregation functions such as we can use them in PivotTable or Subtotal function.