import numpy as np
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

n = 16
fig = plt.figure(figsize=(6,6))
Z = np.zeros((n,n))
Z[::2,::2] = Z[1::2,1::2] = 1
plt.imshow(Z, interpolation='none', cmap=plt.cm.gray, extent=[0,n,0,n], alpha=0.25)
plt.xticks(np.arange(0,n), []), plt.yticks(np.arange(0,n), [])
plt.grid(ls='solid')
delta = 0.01
plt.xlim(1-delta,1+delta), plt.ylim(1-delta,1+delta)
plt.savefig('pylab-grid.png')
plt.savefig('pylab-grid.pdf')