Plotting Frequency Response Magnitude and Phase for first order all pass filter
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.