Today is reportedly the anniversary of the death,
in Paris in 1822, of Jean Robert Argand.
Some related material …
"Wessel's fame as a mathematician rests solely
on this paper, which was published in 1799,
giving for the first time a geometrical interpretation
of complex numbers. Today we call this geometric
interpretation the Argand diagram but Wessel's
work came first. It was rediscovered by Argand
in 1806 and again by Gauss in 1831. ….
Of course it is not unreasonable to call the
geometrical interpretation of complex numbers
the Argand diagram since it was Argand's work
which was influential. It was so named before
the world of mathematics learnt of Wessel's prior
publication. In fact Wessel's paper was not
noticed by the mathematical community until 1895…."
See also Tilting at Whirligigs (Log24 on March 8, 2008)
and The Galois Quaternion.