You are right in that charts are not documented very well.
Go into the query that the chart is based on by doing this:
Open the Report in design view.
If the PROPERTIES window is not alreay showing, right-click on the chart and choose Properties from the shortcut menu.
In the Properties window, click once in the property called RowSource. On the far right, click the build button (the button with the three ... on it). Can you see any field that is SUMMED? If so, you can change the field name like i said above.
If you still need help, from the menu hit VIEW+SQL, copy that code and paste it here.