The cumulative flow diagram is a graphical representation of the issues in different flow states over a period of time. It represents the work accomplished over time. The Burndown chart is drawn for incremental backlog over the span of a Sprint and the cumulative flow diagram is plotted over the span of a release cycle.
As shown in the preceding screenshot, the cumulative flow diagram is plotted displaying the initial "to do" backlog items and how the work is accomplished over time.
You can select Report timeframe for the diagram by selecting a specific date range as shown in preceding screenshot. You can also use the refine functionality to filter the data further.