Math Artifacts

I have a friend who makes quick portraits of his friends for fun. They are sketches that are done in a very short amount of time.

The graph below shows the relationship between the amount of portraits he makes and the amount of time he take to make them. At any given point on the line, the $x$-value is the number of portraits made and the $y$-value is the number of minutes used to make that $x$ portraits.