Decline
Decline
[Cueball is pointing at a line graph at a specific point where it slopes down. The y-axis shows that as y increases, love increases.] Cueball: Our relationship entered its decline at this point. Megan [Outside of panel]: That's when you started graphing everything. Cueball: Coincidence!

Cueball , apparently concerned about the status of his romantic relationship, has constructed a "relationship graph" plotting an ambiguously quantitative metric for love and/or affection against what is presumably time (the x -axis is not actually labeled; ironically in 833: Convincing Cueball states that not labeling graph axes is a relationship deal-breaker). He has identified a sudden drop. Cueball's romantic partner (probably Megan ) notes from off screen that the drop corresponds to the moment Cueball's obsession with graphs began. He claims the two events are coincidental, thereby referencing the recurring xkcd theme of correlation not necessarily implying causation – see 552: Correlation .
The title text references Fourier transformation . The Fourier transform is a technique for discovering the periodic characteristic(s) of a function. A spike at one month on the Fourier transform of the love graph would mean that something happens every month that causes the relationship to change. This is presumably a reference to Megan menstruating, although this isn't proven. This is not something you should mention to your girlfriend and she asks him to stop talking before he finishes the sentence. After this graph the relationship may very well end...
Fourier transformations were mentioned previously in 26: Fourier .
[Cueball is pointing at a line graph at a specific point where it slopes down. The y-axis shows that as y increases, love increases.] Cueball: Our relationship entered its decline at this point. Megan [Outside of panel]: That's when you started graphing everything. Cueball: Coincidence!
