我想绘制概率图，但是imshow会为概率为零的点生成模糊值。如何摆脱围绕真实网格点的模糊外围？

例：

import numpy as np
import matplotlib.pyplot as plt

a=np.asarray([[  0.00000000e+00 , 1.05824446e-01 ,  2.05086136e-04,   0.00000000e+00],
[  1.05824446e-01 ,  3.15012305e-01  , 1.31255127e-01  , 1.05209188e-01],
 [  2.05086136e-04  , 1.31255127e-01 ,  0.00000000e+00 ,  0.00000000e+00],
 [  0.00000000e+00   ,1.05209188e-01  , 0.00000000e+00  , 0.00000000e+00]])
im=plt.imshow(a,extent=[0,4,0,4],origin='lower',alpha=1,aspect='auto')
plt.show()