Visualize the number of "in-flight events" over time


I store events in influxdb with a start timestamp and a duration. How can I visualize the number of events “in flight” over time?

To give some context, you can think of each event as a task - each task having a time when an operator started working on it and the time it took for the operator to finish working on it. What I would like to know is, over time, how many tasks are being worked on at the same time.

Is this doable in influxql+grafana?