While Argand (1806) is generally credited with the discovery, the Argand diagram (also known as the Argand plane) was actually described by C. Wessel prior to
Argand. Historically, the geometric representation of a complex
number as a point in the plane was important because it made the whole idea of
a complex number more acceptable. In particular, this visualization helped "imaginary"
and "complex" numbers become accepted in mainstream mathematics as a natural
extension to negative numbers along the real line.
of 
and the angle 
represents its complex
argument.
