You can use a lookup function to lookup calculate the change against the same month in the previous year. It would be something like this:
(sum([sales)] - lookup(sum([sales]),-12)) / lookup(sum([sales]),-12)
This would work if you have your Months field on your column shelf (I can't tell if you do or not based on your screenshot). You'd also need to include all of the data in your viz, but could use a table calc filter to hide the data without excluding it - lookup(min([date]),0)
If you attach your .twbx file I'd be able to give you a specific response for your situation - it's hard to tell what fields are included on your viz in your screenshot.
Yes if you can attach the twbx file here as an attachment then I'd be able to take a look. That lookup approach will work, but the viz has to have the right fields in the right order so the table calc works correctly.