The graph is not accurate, because GitHub's historical downtime data is not accurate.

For example, here is a Hacker News story about GitHub being down on July 28th 2016: https://news.ycombinator.com/item?id=12178449

Here's GitHub's historical uptime graph (on which this chart is based), saying there was no recorded downtime that day, or in fact that entire month: https://www.githubstatus.com/uptime?page=40