python用matplotlib画图，计算空气质量，饼图显示

import numpy as np
import pandas as pd
import matplotlib
import matplotlib.pyplot as plt

# 读文件，去无用字段
bj_date = pd.read_csv('./beijing.csv').drop(
    ['PM_Dongsi', 'PM_Dongsihuan', 'PM_Nongzhanguan', 'DEWP', 'HUMI', 'PRES', 'TEMP', 'cbwd', 'Iws', 'precipitation',
     'Iprec'], axis=1)
# print(bj_date.head(30))
# 数据清洗
bj_date.drop(bj_date[np.isnan(bj_date['PM_US Post'])].index, inplace=True)


# 空气质量定级
def get_level(pm_us):
    if pm_us < 35:
        return '优'
    elif pm_us < 75:
        return '良'
    elif pm_us < 150:
        return '轻度污染'
    elif pm_us < 250:
        return '中度污染'
    elif pm_us >= 250:
        return '高度污染'


# 给原始数据添加新列表level
bj_date.loc[:, 'level'] = bj_date['PM_US Post'].apply(get_level)
print(bj_date)
# 统计各种空气质量的比列
bj_level=bj_date.groupby(['level']).size() / len(bj_date)
print(bj_level)
#画图
matplotlib.rcParams['font.sans-serif'] = 'SimHei'
l=['中度污染','优','良','轻度污染','高度污染']
plt.pie(bj_level,labels=l,autopct='%.2f%%')
plt.title('北京空气质量指数')
plt.show()