you won't be able to use this variable for further processing. because this gets calculated when the data is retrieved from the database. It's only available for display in a datagrid.
If you'd want to use it in a chart you can retrieve and aggregate the data yourself in a new attribute (or object) at time of creating the graph. Or you can check for a solution where you always pre-process the data whenever a decimal value gets changed.
Larn is right. You can not do it with the aggregation on the datagrid. But you can try the solution in this thread: https://forum.mendix.com/link/questions/96068