Chapter 1 - Linear Functions and Change - 1.1 Functions and Function Notation - Exercises and Problems for Section 1.1 - Exercises and Problems - Page 6: 3

x-axis: time $t$, y-axis: population $P$ in millions.

We want to compete the development of the population $P$ over a century, which is given by the time $t$, here a century. Therefore, x-axis represents the time $t$ and the y-axis represents the number of population $P$.