Scrum burn down charts can they go negative

Question

I work on a small Agile development team which is part of a large, non-agile thinking corporation. Currently, we practise Scrum and occasionally, we exceed our sprint commitment.

My question is, how do you handle burn down charts when you have exceeded your sprint commitment? I can think of two options:

• Extend the y-axis in the negative direction and keep counting down
• Add more cards/stories/work and have the burn down value increase by that amount, burning down when that work is finished.

The ultimate solution for my team is one which is clear to the business and adds real value for the developers. So far, neither of these solutions has worked out perfectly.