This is very difficult and probably not the right tool for the task.
In order to do this you would have to write a script withing ES that computes the concurrency by aggregating on the destination and calculate it for every event.
The easy way to do this is to write the concurrency to the log files when you record the values - so whenever a call starts check if a call already exist (and how many of them exist) and write it in the logs. Then you can easily graph it.
Hope this helps.
-- Asaf.