So in Tableau you need to let it know that you want to do this as an aggregate calculation...
So it would be
the same goes for Margin, Rate of Sales (Units per Store per Week)...well any ratio.
The nice thing in Tableau, is that you have the option...eg. you could use [TY]/[LY], which would be a Row Level calculation, and then put these in bins, and count the products (so you'd get bins with, say, 0-10% Growth, 10-15% growth...etc. (I'm being positive and we don't get negative YoY products!!...but you can, of course have negative bins too).
Hope that helps and makes sense.