View Sub-Tasks in the Burndown Chart

As Stories and other issues can have several sub-tasks underneath the story can only be closed once all sub-tasks are closed. That results in a large gap at the beginning of each Burndown Chart. This also means that this report doesn't give an accurate representation of what work is actually getting done.

Therefore, it would be great if there would be a possbility to choose on which level (Story or Sub-Task) the Burndown Chart is being created or to generally integrate Sub-Tasks into the Burndown Chart.

Thanks,
Irina