"The graphs for each site display both the actual times since last reboot (as an X) and a moving average of uptime over time as a solid green area graph. The colour of the X changes in the event of the site switching operating system. A history of the operating system, web server and hosting location is also provided so it is possible to correlate these changes with the uptime of the site. When we are unable to get a valid uptime measurement for a site, a gap will appear in the plots of the raw data points."
Here’s the graph for blogspot.