Previous versions of Illustrator's graph data labels matched the decimal points defined in the "Cell Style" field. The current version of CC removes any zeros at the end of the label - e.g. "3.00" is changed to "3", or "4.20" is changed to "4.2".

According to Adobe's Customer Care, this is an intended behaviour - but could it be an optional one? Having to manually append zeros to hundreds of labels is not an appealing prospect.