Yes, very familiar with complex numbers, thanks. But, I repeat, you can plot what you want however you want; the question is whether it's helpful, and that will depend on the application. (Suppose the values taken by your dependent variable are all on the circle of radius 1/sqrt(2) centred at (1+i)/2. Then plotting 0, 1, and i collinearly may make a whole lot of sense, though you might actually want to call them -3pi/4, -pi/4 and pi/4 respectively.)