Increment Value (Current - last Value)

I’m using Zabbix as datasource and have an item that gets total of current logged users on RDS servers.
I woul like to create a singlestat panel that shows the value on increment logged sessions, compared to the last measure. For instance, the last value is 30 users and the current is 33, then the panel will show +3 (33 - 30).
I tried to use the Difference parameter in value Stat, but this will be relative to the first register and not to the last (current -first).
Any ideia?