Formatting of Data Labels in Histograms

I have used histograms extensively in MS2003 but have encountered a broblem
using MS2007. I have a spreadsheet that shows dollar amounts of contracts
being awarded during certain months over a three year period. Some months
more than one contract is awarded, others have one or none. Therefore, my
histogram is a stacked column type. My X-axis shows months and my Y-axis
dollars. The problem I am encountering is with the data labels which I put
centered on the column. The dollar amounts show correctly but for some
reason there is a zero at each interval of the X-axis, even for months where
there are no values. There is also a zero at the top of each stacked column!
When I delete the data labels the zeros disappear so I assume it has
something to do with the formatting of the labels but I can't find it.
Any suggestion would be appreciated.