matplotlib的spine_types是什么？

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])

# 设置顶部脊柱的样式和颜色
ax.spines['top'].set_linestyle('--')
ax.spines['top'].set_color('red')

# 设置右边框脊柱的粗细和颜色
ax.spines['right'].set_linewidth(2)
ax.spines['right'].set_color('blue')

plt.show()

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3, 4], [1, 4, 2, 3])

# 隐藏左边框脊柱
ax.spines['left'].set_visible(False)

plt.show()