I was not able to make this work in the data labels but I was able to get the percent difference to appear in the tooltip. I created a calculated field for the MoM percent difference and then added that to the tool tip. Dataset menu I did update the format of the field to show as a percent.
This is really useful to know, but the use case for this graph is for it to be used in a powerpoint presentation, so unfortunately it doesn’t help for this particular case!