Sometimes the capacity and skill set of the team vary drastically from sprint to sprint and even mid-sprint. When that happens you'll want to pay close attention to how the team is doing against the sprint backlog. All else being equal, that's the job of the burndown chart.
The trouble is, it doesn't handle this situation very elegantly at all because it assumes even capacity and that isn't always the case.
Here's a simple way you can get around that problem.