Here's how to read the graphs:
The horizontal axis is the year of the song and the y-axis is the song's popularity according to Billboard's Year-End Hot 100 Singles. Each cell represents a song. The more red a song is, the more often that particular word appears appears in the song. For example, if a song has 5 "love" words and a total of 100 words in the entire song, that song is assigned 5% and a particular shade of color.