Add the ability to see labels on the burndown charts

Problem to solve

The current burndown charts utilize milestones. It shows the number of issues over time as well as the, "weight" of the issues over time. We would like the ability to see a list of labels on the burn down charts as well. This information is already captured on the various team boards. This data is already present. We would like to be able to add another option to sort the burndown chart according to the labels.

For example, we utilize a label called, "Unplanned". These are items that are introduced towards the end of the sprint milestone and they directly affect our ability to meet the milestones. We would like to sort the burndown chart according to the label, "Unplanned" to show how many unplanned user stories were implemented during the milestone.