A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
If I understand your question correctly, you are using a Stacked Column chart?
If you add Series Lines, there will be one line per series between each column. If you only want one line (at the top?) you'd need to have only one series.
But what you could do is add an extra series who's columns are the height of the sum of the other series... Then you can move the new series to the secondary axis, then with that series selected apply Series Lines to the chart. You can then format the line and fill formatting for the new series to no fill/line to hide it, then delete the legend entry by clicking once, then once again on the legend entry and then pressing delete.
Make sense?
Cheers
Rich