You will need to change the graph to an advanced graph
1. right mouse click on the chart and select Chart Expert
2. Select Data tab
3. Click the advanced button
4. Select the field that you have grouped and move to the On change of list box
5. Add the fields to be summed to the Show Values list box.
Then you are done.
Note You can only list 1 on change of field with multiple Values, or multiple on change of with one value...