It's one of the misunderstood pieces of scrum to think that burndown charts should be linear. If a team of 5 has established a velocity of 65 story points / 2-week sprint, and they accept 5, 13-point tasks for the next sprint, you'd expect a burndown chart that would look almost horizontal for the entire sprint until the very end, when it would drop vertically to the x axis. You might get linearity if there were 65, 1-point tasks...
I think you're getting at the heart of why burn down charts are so pointless. The shape changes sprint to sprint, depending on what kind of stories you take on. If the sprint is entirely high pointed stories, your chart will be mostly horizontal. If the sprint is entirely 1 pointers, the graph will look like an exponential decay.
Since the shape is always different, there's virtually zero useful information to take away from it.
Some people in this thread claimed that burn down charts are useful for finding out a team's capacity. I disagree - you don't need a chart for this. If your team has a target velocity of 30 points and routinely fails to meet it, reduce the target velocity. Likewise, if the team routinely completes all of their work well before the end of the sprint, slightly increase the target velocity.
My insistence is always that what happens inside the Sprint is the concern of the development team and nobody elses.
However it's important to remember that the team itself should set their velocity in negotiation with the product owner. This shouldn't be set by some external force.
