I'm not sure what you intend the correct number to be. You're doing the cube root of negative numbers. They're going to be complex.
By default, plot() will only plot the real component of complex inputs. If you're expecting the plot to be symmetrical, consider plotting abs(y) to get the magnitude of the complex-valued region.
x = -5:0.1:5;
y = x.^(1/3);
h1=plot(x,real(y)); hold on