Dynamics of burn down charts in Scrum

I have a question about how dynamic the Y axis of a burn down chart in Scrum should be. We plot the chart in the beginning of a sprint having the total number of estimated story points on the Y axis, and the planned days on the X axis.

Usually, during the sprint, we have a fair amount of:

• unplanned tasks / stories;
• tasks / stories that take longer than estimated (re-estimated by the person checking out the task);

Questions:

• should the story points of the unplanned tasks be plotted into the chart? if so, extend the y axis as well and redraw the expected curve? or just plot the points and have an actual curve with points maybe higher than the starting point?

• should the re estimations be counted when plotting the chart, or just the initial estimations? same questions as for the first question...

I would prefer to ignore the unplanned items and the re-estimations as they will show up in the actual focus factor calculation anyway. Is it wrong?

Try using a burn UP chart.

http://www.nearinfinity.com/blogs/lee_richardson/forget_burndown_use_burnup_charts.html

Also, I would do everything in your power to stop the unplanned items. They are typically very caustic. If it's code debt cashing in, try to address it a little bit at a time in every sprint. If it's a consistent amount of time every sprint, perhaps create a story at the start of the sprint for "unplanned tasks" or "production fixes" or something like that.

In the end, what really matters is that the burndown chart allows you to track progress (or lack thereof) toward the commitment. So as long as you're achieving that, you're good to go. Which means, really any of these solutions would work - just pick one and go with it.