Exel - add vertical line to two axis graph

[Mia11]

I have created a graph with two y-axis and one x-axis. One data series is displayed as a column and the other one as a line. The x-axis shows elevation, the primary y-axis precipitation and the secondary y-axis the number of grid points. I need to add to those data a vertical line that crosses the x-axis at a specific elevation. All the information I was able to find shows how to add vertical lines (target lines) to graphs with one axis but I couldn't find anything that shows how to do this when I have already a secondary y-axis. I work with Excel 2003. I would really appreciate your help.