Cumulative Flow diagram is a very good starting point for stop-the-line or retrospective meeting. Here is a real example from TargetProcess development.
You can see in the chart that we had a bottleneck in the beginning of December. It was caused by a quite complex user story. In theory a single user story should not affect cycle time significantly and should not create bottlenecks, but if you violate some rules it might be the case.
Unfortunately, after the merge quite many bugs were found in the build during smoke testing. These bugs took more than a week to fix, and during this time we were unable to release anything, since the merge was already done and the rollback was quite complex as well.
The lesson learned is to put more effort into testing of user stories that are more complex in nature. This particular story affected many places in the application and usual smoke testing was not enough. So we are going to introduce a new class of service (let's call it “technically complex story” so far 🙂 which would mean more in-depth testing and verification before the merge.
In general, Cumulative Flow chart is a valuable tool to analyze historical data, but for emergency bottleneck identification Kanban Board is even better. If you have limits on Kanban Board, you see problems immediately.