This works great!
Compute the sum in the worksheet. Copy this range, select the chart, and use Paste Special to add it as a new series. This makes a new column stacked atop the others, not what you want. Select this series, go to Chart menu > Chart Type, and choose a line type chart. Double click the line, on the Data Labels tab, choose the Show Value option, and on the Patterns tab, choose None everywhere so the series becomes invisible. Finally, double click on the series of labels, and on the Alignment tab, choose Above for Position.
Jon Peltier, Peltier Technical Services, Inc.