I used this to generate burn-down charts from my Trello board.
You have to have boards with word "Sprint" in the name. The script will find those, and allow you to choose ones to report on.
It only work for tasks that have been moved to a column called "Done" ONCE.
Good luck! :-)
It works for me. You will need to edit the index.html file and insert your Trello API key there.