axis:即轴
axes:灵活的subplot，可以翻译成轴域，subplot即特殊的axes.

subplot

fig = plt.figure 
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212)
plt.show()

axes

fig = plt.figure()
ax3 = fig.add_axes([0.1,0.1,0.8,0.8])
ax4 = fig.add_axes([0.72,0.72,0.16,0.16])
plt.show()