Hi Meenu, it works great, thank you very much.
I should "fix" first purchase and last purchase to user as well, shouldn't I?
My idea is to calculate each customer rhythm as Datediff (days, first purchase, last purchase)/(Count (number of records)-1). So I'm thinking that in the Viz in page 1 I should be just fine using First purchase and Last purchase as they are right now, but that I should use the LOD expression for other Viz's.
Nevertheless, thank you very much, the solution works perfectly.