Title text: 'There is also a spike on the Fourier transformation at the one month mark where--' 'You want to stop talking right now.'
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 time. 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 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's menstrual cycle, that is, her 'time of the month,' stereotypically depicted as having a woman uncharacteristically crabby or moody at that time. 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!