Yes you can very easily.
Open the report or form in design mode that contains your chart.
Right click on the chart and select Properties.
On the Row Source click on the button with the three dots at the end of the SQL statement which will open the SQL as a conventional query
In the field that you have set to sum insert A: before the actual field name in the grid and then close the query clicking on Yes to save the updated SQL. Your category will now appear as A