Here is a first attempt at putting footers in a Spark DataGrid. The approach taken is to customize the skin and add an additional Grid element and assign it an custom ArrayList that contains the summary data. A subclass of GridColumn allows specifying a summary function. In the demo, the first column does a sum, the second does an average.
One of the surprises was in the partAdded handler. Because you can’t control the order in which skin parts are added in subclasses, you have to build in some redundancy when there are interdependencies between skin parts.
Usual caveats apply.