Select Series tab.
Click on Add button
For Values select the column of summed data
Now the summed data series sits stacked on top of your other stacked data.
Right click this added series on your graph.
Format data series
Select Axis tab
Click Secondary axis radio button
Now the summed data series sits on top of, and is hiding, your stacked data.
Right click on this series (again).
Select Add trend line
Chose the type you want
Your stacked data is still hidden on the graph, but now you have a trendline for the summed data.
Right click on series
Select Format Data Series
Select Patterns tab
Click None for both Border and Area radio buttons
If you have a legend you will have to select the summed data series and delete it, and you should be good to go.