set numerics complex
set sample grid 400x400
set nogrid
set size square
set key below

set c1range[0:2]
set colmap hsb(c1):(c2*0.7+0.3):1

f(x) = 3*x**2 / (x**3+1)
plot [-3:3][-3:3] abs(f(x+i*y)):arg(f(x+i*y)) with colourmap