IIR filter freq response

import numpy as np
import scipy.signal as dsp
import matplotlib.pyplot as plt

b = np.array([0.0089, -0.0045, -0.0045, 0.0089])
a = np.array([1, -2.5641, 2.2185, -0.6456])

w, h = dsp.freqz(b, a)

fig = plt.figure()
ax1 = fig.add_subplot(211)
plt.title('Digital filter frequency response')

plt.plot(w, 20 * np.log10(abs(h)), 'b')
plt.ylabel('Amplitude [dB]', color='b')
plt.xlabel('Frequency [rad/sample]')

ax2 = ax1.twinx()
angles = np.unwrap(np.angle(h))
plt.plot(w, angles, 'g')
plt.ylabel('Angle (radians)', color='g')
plt.grid()
plt.axis('tight')

ax3 = fig.add_subplot(212)
gdelay = -np.diff(np.unwrap(np.angle(h)))/np.diff(w)
gdelay = np.append(gdelay, gdelay[-1])
plt.plot(w, gdelay)
plt.ylabel('Group delay [samples]', color='b')
plt.xlabel('Frequency [rad/sample]')
plt.grid()
plt.axis('tight')
plt.ylim(ymin=0)
plt.show()