I am assuming that you are not using a dual-axis chart and just placing more than one measure on the Labels shelf.
There is no direct way to center the labels if you have a single bar chart(i.e not a dual-axis graph)
Follow the steps to get a centered label using a dual axis graph:
1. Create a dual axis chart by replicating your measure value in the rows shelf.2. Put the label you want to be centered onto the second set of bars
3. Format the positioning of the label in the second graph-> make the vertical alignment-> center
Hope this helps,
Thanks Soham. That worked.