Plotting Frequency Response Magnitude and Phase for first order all pass filter Having trouble plotting the frequency response characteristics for first order all pass filter. The Magnitude is expected to be constant across entire freq and Phase is expected to be only decreasing (with jump from -pi to pi). I don't see either of this happening, provided plot from octave.
$H_{ap}(z) =  {z^{-1} - a^* \over 1 - az^{-1}}\tag{1}$
Above is condition to enable all pass filter.
code.m
num = [0.8,1]  
den = [1;0.8]   
TF = tf(num,den)   
[zz,pp,kk] = tf2zp(TF)  
figure(1);  
bode(TF);  
figure(2);  
pzmap(TF); 

Octave output:
Octave output:

Transfer function 'TF' from input 'u1' to output ...

      0.8 s + 1
 y1:  ---------
       s + 0.8

Continuous-time model.
zz = -1.2500
pp = -0.80000
kk =  0.80000
As pointed out by Hilmar in a comment, you should remain in the discrete domain. This is most easily done by using the functions freqz() and zplane():

b = [.8,1]; a = [1,.8];
[H,w] = freqz(b,a,2048);
subplot(2,1,1), plot(w/pi,abs(H));
subplot(2,1,2), plot(w/pi,angle(H));
figure, zplane(b,a)

Note that for a first-order all-pass filter it's also quite straightforward to come up with analytical solutions for the magnitude and phase of the frequency response.