That graph is from updown.io – Website monitoring, simple and inexpensive which only measures the response time from the server, to detect if it is down or not, but time-to-render is a valid concert too to keep users satisfied. In fact Apdex seems to be a universal metrics for that Apdex - Wikipedia
I like the design that updown.io uses to explain the concept - lines and colors, but the logical part of the diagram is hard to read, so I wonder if it can be improved. Like, what professors in visual communication would do with it. 