This does not describe a proportional function because $\displaystyle{y}\usepackage{cmbright}\Huge\$ does not equal a constant times $\displaystyle{x}.\usepackage{cmbright}\Huge\$ For example, when $\displaystyle{x}={1},{y}={2}={2}\cdot{1},\usepackage{cmbright}\Huge\$ but when $\displaystyle{x}={2},{y}={5}={2.5}\cdot{2}.\usepackage{cmbright}\Huge\$