How to create multiple x-axis labels?

[potentenum]

Right now I have this:

Screen Shot 2013-11-29 at 1.02.47 PM.jpg

How can I put another label under each bar (NOT outside end or inside end or any additional label. This is an actual label? For example, under the first blue bar labeled 56.7%, I also want to put N = 50 below the X-axis, above "Optimal." I also want to put N = 25 for the red bar immediately next to it labeled at 75.7%.

This is not a label, as it is below the X-axis.

An example of this can be found here (scroll down for second chart)

Thank you!

http://link.springer.com/article/10..../fulltext.html

[Andy Pope]

You can use 2 additional series and put them on the secondary axis.
The use their data labels linked to cells to display the base sizes.

Use a custom number format to hide the negative % axis values.