You've to work with the format of your data or you can use Custom SQL to achieve the output.
Find my approach, I've connected your excel with legacy connection used custom sql query to transpose your data. Doing that I can get the Category and their sales by year.
select * from
(SELECT  AS Category,[Measure 2015] AS [Sales], "2015" AS [Year] from [Sheet1$]
SELECT ,[Measure 2016], "2016" from [Sheet1$])
where [Category] is not null
Then go to worksheet and drag the fields like that. Create a calc field to get the difference
ZN(SUM([Sales])) - LOOKUP(ZN(SUM([Sales])), -1)
Hope this help. Let us know If you've any query. workbook (version 9.3) attached for your reference.
Test.twbx 50.3 KB