How it is formated your data? Just to replicate here.
It depends on how your data is structured, but you could create a column for May 2016 and May 2017 e.g.:
(I'll be using 2012 and 2013):
and another for May 2013
Then I've added these to rows and converted them to DISCRETE:
Then I created a % Difference field:
Put this field into Columns and formatted as percentages:
Thanks, I've attached the workbook so that you can review.
Bars v2_v10.2.twbx 7.5 MB