What are the green and red gems beside Github Gist Revisions

I have just started to use Github and have been doing some tests with Gist, I notice beside each revision there are little green and red dots of varying amounts. Does anyone know what they are?

The following image, for example, is captured from this gist.

Those red and green dots represent insertions and deletions in the revision, respectively. There is something similar associated with commits on GitHub (actually, you see the same information when committing in Git), except there you can hover the mouse pointer over the dots and get more information. For example, when hovering over the colored squares in this commit you can see the number of additions and deletions.