During a sprint, a Scope Change occurred, meaning a Story was either added, removed, or changed in value during the course of the sprint. The GreenHopper Burndown chart calculates points as though the change was made at the beginning of our sprint.
For GreenHopper 5.9.2 and earlier, this is normal program behaviour.
GreenHopper 5.9.3 changes this behaviour to calculate the chart based on daylong increments, not sprint-long, and fixes this issue. To resolve the issue, download and install GreenHopper 5.9.3 either through your Plugin Manager or from plugins.atlassian.com. Be sure to review the GreenHopper Upgrade Notes prior to starting your upgrade.