An Argand diagram is the graphical representation of complex numbers written in polar coordinates. For example, if $z\in \mathbb{C}$ is a complex number, then $z$ can be written as $re^{i\theta}$ , where $r$ is the length of $z$ considered as a vector $(x,y)\in \mathbb{R}^2$ , with $z=x+yi$ , and $\theta$ is the value such that $\tan \theta = \frac{y}{x}$ , and can be interpreted as the angle $z$ makes with the $x$ -axis. The Argand diagram of $z$ is thus

Argand is the name of Jean-Robert Argand, the Frenchman who is credited with the geometric interpretation of the complex numbers [Biography]