I made a graph of my past relationships.

This joke gets its chuckle from a classic case of wordplay, specifically a pun that twists scientific terminology into a relatable relationship commentary. The humor mechanism hinges on the auditory similarity between "ex axis" and "x-axis," and "why axis" and "y-axis." In mathematics and science, an x-axis typically represents an independent variable, while a y-axis represents a dependent variable, allowing us to plot data and observe relationships between different factors.

The genius here is how the joke redefines these fundamental graphing components. Instead of plotting data points, our imaginary graph is charting the tumultuous landscape of past romances. "Ex axis" perfectly encapsulates the presence of former partners – the "exes" – who are a constant feature in one's romantic history. The "why axis" speaks to the universal, often unanswered question of "why" relationships end, why certain things happened, or why we even got involved in the first place. It’s a clever way to apply the analytical framework of science to the inherently unscientific, emotional realm of human connection, making light of the often-complicated process of reflecting on love gone by.