Error in an accumulated measure

[aheavy95]

HI all,

I am trying to create a line chart that will show the number of users who have a birthday field in the DB by year-month accumulated.

when I create a simple calculation:  Count(UserBirthDate) - I get the right number  

The problem starts when I add the year-month field and it causes duplication in the data as the year-month is associated with the sales. 

The expression in the linechart: 

For example  for this user-key the Count(UserBirthDate) shows 1 as it should since I only chose one user who has a birthdate in the DB, but  in the linechart the amount increases in each month the user had a purchase all the way up to 7 in this case

How can I solve this issue so I will get only 1 Count(UserBirthDate)  per user even when adding the yearmonth dimension?

Thanks