Adding secondary vertical axis to chart in Excel 2010

Hi folks:

I am trying to add a secondary vertical axis to a chart in Excel 2010. I have two data series.

I have read that by clicking on the second data series -> format data series -> series options, there should be an option for adding a secondary vertical axis. However, I do not see that option when I get to series options - only gap depth and gap width.

Thanks for your help!

Sorry for the confusion - I see that I need to select a different chart type for the second data series. Now I see the option to add a secondary axis.