y does not equal a constant times x. For example, when $\displaystyle{x}={1},{y}={6}={6}\cdot{1},\usepackage{cmbright}\Huge\$ but when $\displaystyle{x}={2},{y}={6}={4.5}\cdot{2}.\usepackage{cmbright}\Huge\$ Also, in order to be a proportional function, when $\displaystyle{x}={0},\usepackage{cmbright}\Huge\$ $\displaystyle{y}\usepackage{cmbright}\Huge\$ must be $\displaystyle{0}\usepackage{cmbright}\Huge\$ as well.