Definition:Argand Diagram/Historical Note
Historical Note on Argand Diagram
The Argand diagram appears in Jean-Robert Argand's self-published $1806$ work Essai sur une manière de représenter les quantités imaginaires dans les constructions géométriques (Essay on a method of representing imaginary quantities by geometric constructions).
This would have passed unnoticed by the mathematical community except that Legendre received a copy.
He had no idea who had published it (as Argand had failed to include his name anywhere in it).
His brother Jacques Français found it in his papers after his death in $1810$, and published it in $1813$ in the journal Annales de mathématiques pures et appliquées, announcing it as by an unknown mathematician.
By this time, however, Carl Friedrich Gauss had already himself invented the same concept.
It must be noted that this concept had in fact been invented by Caspar Wessel as early as $1787$, and been published in the paper Om directionens analytiske betegning by the Danish academy in $1799$.
Wessel's precedence is now universally recognised, but the term Argand Diagram has stuck.