Hi
Our Burndown chart shows issues completed as we work in the sprint, however when we close the sprint at the end, the redline becomes flat, and no issues are showed as completed. Only any issues that were not completed show in the reports. The other charts such as the velicity chart display the same behaviour.
A couple of examples are below. The firrst image is a closed sprint, the second is the current sprint in progress. As you can see from the in progress sprint, everything looks fine. However when we close it, it will end up looking like the first one.
FYI: Our "Done" comlumn in our sprint board is "Closed" status.

