The best tool, though you could argue about the free part, for me is a whiteboard or flip chart and a marker. Though any sheet of paper and pen will do.
It's easy to implement, too. Just put numbers on your cards, draw the two axes and you're set.
Start from there and get your process on order first. Once you and your team have established your own flavor of scrum, you will know what
your requirements are and screening tools will be a quick exercise, but not knowing what the tool is supposed to do, flexibility is key. And not much can beat board and marker in that respect.
Source: my own experience building and coaching scrum teams