How do I universally change the background of a data label in grap
 

I have a series of graphs in a single worksheet. Somehow, all data labels
have begun to print with a white (clear) background against the gray plot
area. I must have somehow universally changed the data label format, but I
don't know what I did, and I now need to change them back (to transparent ?).

This change must have been to Excel itself rather than to the specific
graph, because even previous graphs now open with the data labels having a
white background.