The Iteration Burndown Chart is a graphical representation and projection of remaining unfinished work in that Iteration. In an Iteration, the Burndown Chart is located in the Reports tab of the Iteration Detail Page.
The start date and end date of the Iteration Burndown Chart correlates to the start date and end date of the Iteration.
Understanding the Burndown Chart
Actual Stories/Points Remaining is represented by the bold blue line and references the previous day's points/stories remaining, plus points/stories added, minus points/stories removed, minus the number of points in stories completed.
The ideal number of Stories or Points remaining is represented by a diagonal, grey straight-dotted line, starting from the actual stories remaining on the first day of the iteration, and falling to 0 on the last day of the iteration.
Stories/Points Added is represented by the purple bar and references when points in a story are adjusted up, or when a story with points is added to the iteration.
Stories/Points Removed is represented by the yellow bar and references when points in a story are adjusted down or when a story with points is removed from the iteration.
Stories/Points Completed is represented by the light green bar and references that day's number of stories or points completed.
The colors on the graph showcase activity in the burndown. The easiest way to understand the graph is to notice how the colors affect the bold blue line (Actual Stories/Points Remaining). For example:
In the image above, we see that the green bar seems to sit on top of the blue line in the chart. The green bar represents stories/points completed; this is designed to reflect that the stories completed on that day have pushed down the blue line to where it is currently at. Conversely, the purple bar--which represents points added--is meant to reflect the addition of points or stories have contributed to the blue line being pushed up to where it is currently at.
The setting for whether your Iteration's Burndown Chart displays its data in either Story Counts or Points can be configured in via the Story Count/Points drop down at the top of the Burndown Chart:
An Iteration's Burndown Chart will "freeze" once its end date has passed. If you are viewing your Iteration Burndown chart in Points, the Burndown Chart will reflect the current Points of the corresponding Stories, up to the last day of the Iteration. That is, if the Points Estimate of a Story in an Iteration changes after the Iteration has ended, it will not be reflected in the Burndown Chart. Similarly, if you are viewing your Iteration Burndown chart in Story Counts, the number of Stories added to or removed from the Iteration after the Iteration has ended, will not be reflected in the Iteration's Burndown Chart.