How do you plot #5i#?
I tried this:
We can use the Complex Plane (aka, Argand-Gauss Plane) where on the horizontal axis you plot the real parts of a complex number while on the vertical one you plot the coefficient of the imaginary part.
So in your case you have a complex number in the form:
the real part is zero while the imaginary is
The dot represents your complex number on the Complex Plane.