I had this thought on my mind considering the "meaning" of complex numbers in terms of them being graphed on a plane.
It is obvious that complex numbers "should" be graphed with its real part on the x-axis and its imaginary part graphed on the y-axis, but how did someone arrive at the conclusion that it is the "best" way to graph them. This reasoning makes statements such as Euler's Formula and the concepts such as "modulus" and "argument" understood since they display clear geometric meaning.
The question is: How did someone reason that complex numbers are graphed the way they are on a plane? Why did they decide so? What is so special about the imaginary unit that gives complex numbers geometric meaning on the plane?