at the end of your code I added the following:
covN = c/N;
The first plot uses the entire frequency array and uses fftshift to put the cmag values at the appropriate location.
If you want to use both positive and negative frequencies, i'ts more in the spirit of the thing to plot the real and imaginary parts instead of the absolute value. When this is done you can see that the imaginary part is an odd function of frequency. The real part is an even function, but since you are transforming a bunch of sines, the real part comes out zero as shown on the plot.