The vertical axis shows the date that the book or movie is supposed to be set in. So futuristic books and movies like, “The Time Machine,” “1984,” and “Star Trek” appear toward the top of the graph, while historic tomes like “The Book of Genesis,” “Julius Caesar,” and “The Ten Commandments” appear at the bottom. “2001: A Space Odyssey” makes it into the very top and very bottom of the graph, due to its vignette at the dawn of man. (If things are still looking weird to you, it might be because the axes are logarithmic, a common practice in economics that condenses really big or small values for ease of viewing.)
The diagonal line across the bottom indicates period pieces that are now so old they are closer to their historic source material than to today’s date. For example, "The Last of the Mohicans" was written in 1826 about events that occurred in 1757. This may come as a shock to aging Millennials, but that latter category now almost includes “I Love the ‘90s” (which aired in 2004) and "Freaks and Geeks" (1999-2000).