Kibana – How can I visualizie the number of running tasks

I’m experimenting with Kibana (ELK Stack) at the moment and I’m wondering whether or not this is possible:

I have data which contains a timestamp which is the start of a task and a field which gives the duration of that task in seconds. I could as well include the end-timestamp but at the moment I don’t have that.

Now I want to visualize the amount of active tasks at a given time. It was no problem visualizing the number of started tasks at a given time.

I did this with a “SUM of SUCCESS” (Success is a field containing 1 for a task which succeeded) for the Y Axis. For the X-Axis I used “Date Histogram” for the timestamp field (“created_at”).

But how can I tell kibana to count “active tasks”, meaning that entries have to be counted as well for every point in time which falls into the timeframe of start-time until start-time + duration?