F.e. you select year 2014, there will be expression only(year)-1, that will handle it,
sum({<year={$(=Only(year))}>} Sales) //First expr that calculates selected year,
sum({<year={$(=Only(year)-1)}>} Sales) //Second expr that calculates year-1,
='Sales '&Only(year_sale) // First expr to label calculation for selected year,
='Sales '&(Only(year_sale)-1) // Second expr to lavel calculation for year-1,
For ease of use you can on general tab of chart properties identify Calculation condition, f.e. Count(DISTINCT year_sale)=1
This means, that chart will shown up only when you select needed (starting) year which will be the basis to count previous year.
Don't forget to set ERROR message that will helps the user choose the year, go to Error messages on General tab of chart properties, and further locate Calculation condition unfulfilled and set reminer for something like that: Please select sales year.
User will see the message in chart, further select the year 2015, and he will see results for 2015 and 2014, if he selects, f.e. 2013, he will see 2013 vs. 2012.
