Nevermind, I just figured it out!

I clicked the data label on the chart, then put the formula in the formula bar, and, voila.
 Hi,

 I have a bar chart that shows projected and actual numbers for four quarters.

 I'd like to label each pair of bars with the PERCENTAGE between the projected and actual numbers for each quarter, rather than the actual number each bar represents.

 Is there a way to do this based on a formula?  (As opposed to creating text boxes and insertng them?)

 Thanks so much