Calculation of comulative sum of unique ids

[#1]

Hi,

Assume I have a table that looks like:

UserID	ActivityTime
a1	10/02/2015
a2	10/02/2015
a3	10/03/2015
a1	10/03/2015
a5	10/04/2015
...

Which represents the timestamp when Users made some activity. Now Assume that I want to create a line chart that on X axis is ActivityTime and on Y Axis is the number of unique Users who made some activity the last 30 days on the date shown in X Axis.

One way I thought about was to use Expression like:

Rangesum(above((Aggr(count(distinct UserID),  [ActivityTime])),0, 30))

However this expressions is not exact. For example if user a1 made activities both of 2nd and 3rd, the above expression counts the user twice.

Any recommendation on how to create the proper expression is appreciated.

Thanks a lot,

Shahab