Im trying to see some information display in a way like this:
I would like to see also, the purchases that this people made three months ago. Like this:
Sales Purchases three months ago
January 10 x
February 20 x
March 30 x
Is it posible to create a calculated field where i can choose to see this particular information?
It is possible... although it depends on exactly what you want the x to show (last value, number of purchase, previous purchase date?)
Check out Number 10 in this link (you'll need Desktop v9 or higher) >> Top 15 LOD Expressions | Tableau Software
If you get stuck give us a shout!