There's not really a way that's built into the chart to do this, but you can
accomplish the aggregation on the sheet, then link a text box to it.

For example, if your chart data is in cells A1:A4, enter in =SUM(A1:A4) to
cell A5. Then insert a text box on top of the chart (Insert > Picture >
Autoshapes). Select the textbox and in the formula bar type =Sheet1!A5

The textbox will be linked to that cell. You can add a label to it too, to
by entering ="Total: "&SUM(A1:A4) or something like that to cell A5.