You can use a RANK_UNIQUE formula to calculate the sorting per year:
Then put the Ranking between the Year and Cust fields and make sure you compute the ranking using the Cust field. You can then hide the Ranking header from the table:
However, 2015 will have a different ranking than 2016. I'm not sure if this is the desired behaviour or that you would hide 2015 from the viz anyway.
Thank you both of you
The Rank_Unique works quite well and is simple to do.
And yes, I will hide 2015 data, so no issue with it.
Thanks again !