In case of your code, the X axis of the plot is just the index of the frequency response vector. If c_f is a 8000x1 vector, then the X axis of the plot will go from 0 to 8000.
I think it would be better if you were to use FREQZ to return both the frequency response and the corresponding frequency vector:
[ c_f , w ] = freqz( c_w );
plot( w , abs( c_f ) );
The X axis now will be in radians per sample.
