How to create a cumulative visual for count of strings over period of time?

I have a field with the data type of a string. I want to get the count of this string field, per day, and accumulate it over a month. How can i do this?

fields are processing_date (date), casereference (string). Visual should show and accumulated count of the casereference over a period of a month.

And if possible, after a month how can i drop it back down to 0, and restart on each new month?

runningSum(distinct_count({casereference}),

[{processing_date} ASC])