下面是我在for循环中使用函数axhline的方法，你可以用文档中的参数调整行：

import matplotlib.pyplot as plt
from matplotlib.cm import ScalarMappable
from matplotlib.colors import BoundaryNorm

plt.plot() # dummy Axes

nmin, N = 0, 11
bounds = [nmin-0.5]+[n+0.5 for n in range(nmin, N)]
norm = BoundaryNorm(bounds, 256)
cm = plt.get_cmap('cool')

cb = plt.colorbar(ScalarMappable(norm, cm), format='%02d', ax=plt.gca())
cax = cb.ax
for bound in bounds:
    cax.axhline(bound, c='k', linewidth=0.4)
cb.set_ticks(range(nmin, N))

plt.show()

图：