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...