Create vertical line for today

I need to create a vertical line on a Gnatt Chart that shows today's date. I have tried several solutions and it does not work

I almost got it but I had a problem with this UTube video.
http://www.youtube.com/watch?v=EuWzDDkXIQ8

Everytime that I opened the Format Data Series dialog box ctrl+ 1 when I selected the data it would not let me plot the series on the secondary axis. How do I change that so that box is not greyed out? Once I do that it should work.

Al