Plotting a harmonic function

Hi, I am trying to plot the given function:

syms x
Z = x.^2 - exp(x)/3 + (2*exp(-x/2)*sin((sqrt(3)*x)/2))/sqrt(3) + 4/3*exp(-x/2)*cos((sqrt(3)*x)/2)
fplot(imag(Z),   [ -4, 4 ])
figure(1)

however, it appears as a straight line, while in WolframAlpha it's quite something different.

Can someone see if this looks ok?