matplotlib分面子图中如何为饼图添加黑色矩形外边框

使用matplotlib绘制2行2列分面图时，3个堆叠柱状图子图默认带有黑色矩形外边框，但首个饼图子图无此外框，需要统一子图样式，让饼图也显示相同的黑色外框。

附问题复现代码：

import pandas as pd
import numpy as np
pd.set_option('max_columns', None)
import matplotlib.pyplot as plt
import matplotlib as mpl

# 数据
data = {
         'type_sale': ['g_1','g_2','g_3','g_4','g_5','g_6','g_7','g_8','g_9','g_10'],
         'open':[70,20,24,150,80,90,60,90,20,20],
         'closed':[30,14,20,10,20,40,10,10,10,10],
        }

df = pd.DataFrame(data, columns = ['type_sale','open','closed'])

data1 = {
         'type_sale': [ 'open','closed'],
         'structure':[70,30],
                 }
df1 = pd.DataFrame(data1, columns = ['type_sale','structure'])

# 绘图
labels = ['open','closed']
fig, axs = plt.subplots(2,2, figsize=(10,8))
plt.subplots_adjust(wspace=0.2, hspace=0.6)
df1.plot(x='type_sale', y='structure',labels=labels,autopct='%1.1f%%',kind='pie', title='Stacked Bar Graph by dataframe',ax=axs[0,0])
df.plot(x='type_sale', kind='bar', stacked=True, title='Stacked Bar Graph by dataframe', ax=axs[0,1])
df.plot(x='type_sale', kind='bar', stacked=True, title='Stacked Bar Graph by dataframe',ax=axs[1,0])
df.plot(x='type_sale', kind='bar', stacked=True,title='Stacked Bar Graph by dataframe', ax=axs[1,1])

plt.suptitle(t='Stacked Bar Graph by dataframe', fontsize=16)
plt.show()

效果参考：

matplotlib绘制饼图时默认会隐藏坐标轴外框，在饼图绘制完成后，为对应子图添加如下代码即可开启外框显示，边框默认样式和其余柱状图子图完全一致：

axs[0,0].set_frame_on(True)

import pandas as pd
import numpy as np
pd.set_option('max_columns', None)
import matplotlib.pyplot as plt
import matplotlib as mpl

# 数据
data = {
         'type_sale': ['g_1','g_2','g_3','g_4','g_5','g_6','g_7','g_8','g_9','g_10'],
         'open':[70,20,24,150,80,90,60,90,20,20],
         'closed':[30,14,20,10,20,40,10,10,10,10],
        }

df = pd.DataFrame(data, columns = ['type_sale','open','closed'])

data1 = {
         'type_sale': [ 'open','closed'],
         'structure':[70,30],
                 }
df1 = pd.DataFrame(data1, columns = ['type_sale','structure'])

# 绘图
labels = ['open','closed']
fig, axs = plt.subplots(2,2, figsize=(10,8))
plt.subplots_adjust(wspace=0.2, hspace=0.6)
df1.plot(x='type_sale', y='structure',labels=labels,autopct='%1.1f%%',kind='pie', title='Stacked Bar Graph by dataframe',ax=axs[0,0])
# 开启饼图子图外框
axs[0,0].set_frame_on(True)
df.plot(x='type_sale', kind='bar', stacked=True, title='Stacked Bar Graph by dataframe', ax=axs[0,1])
df.plot(x='type_sale', kind='bar', stacked=True, title='Stacked Bar Graph by dataframe',ax=axs[1,0])
df.plot(x='type_sale', kind='bar', stacked=True,title='Stacked Bar Graph by dataframe', ax=axs[1,1])

plt.suptitle(t='Stacked Bar Graph by dataframe', fontsize=16)
plt.show()

内容的提问来源于stack exchange，提问作者silent_hunter