That is simple and extremely helpful, thank you! I can't believe I haven't stumbled across that feature yet.
What about attempting to compare my sales ex: (2015 vs 2016) I want to show shading on a map for % of change. I was even thinking I'd like to create buttons so the dashboard will automatically complete the calculations depending on what two sales years the user selects. I'm not sure this is even possible in Tableau.
Everything is possible in Tableau!
Right click on your measure, quick table calculation > percent difference > compute using > [Year]
This will give you year over year growth. Alternatively, you could use parameters to allow your users to select 2 comparison years.
For example, parameter 1 could be:
CASE [Parameter 1]
When '2016' and YEAR([Date]) = 2016 then [Measure]
When '2017' and YEAR([Date]) = 2017 then [Measure]
Essentially the same for Parameter 2. Your YoY growth calculation is then [Calculation from Parameter 1]/[Calculation from Parameter 2] which would give you % increase/decrease.
Does that help?