为什么未显示仅显示图例名称的choropleth映射（Python）？

Question

我实际上试图显示一个未显示的Choropleth映射位，仅显示带有图例名称的灰色空间...我什至检查了key_on参数。
我什至尝试将其另存为HTML文件并在Microsoft Edge中打开它，但它无法正常工作。

   import pandas as pd
   import matplotlib as mpl
   import matplotlib.pyplot as plt
   import wget

   df_c=pd.read_csv('Police_department_Incidents.csv')
   df_crime=df_c.groupby('PdDistrict',axis=0)['PdDistrict'].count()
   df_crime=df_crime.to_frame()
   df_crime.columns=['0']
   df_crime.reset_index(inplace=True)
   df_crime.columns=['Neighborhood','Count']
   df_crime


   print('Beginning file download with wget module')

   url='https://cocl.us/sanfran_geojson'
   wget.download(url,'c:/Users/pj/san_fran.json')

   loc_json=r'san_fran.json'
   crime_map=folium.Map(location=[-122.431297,37.773972]
                        ,zoom_start=12,tiles='Mapbox Bright')
   crime_map.choropleth(geo_data=loc_json,
                        data=df_crime,
                        columns=['Neighborhood','Count'],
                        key_on='feature.properties.DISTRICT',
                        fill_color='YlOrRd',
                        fill_opacity=0.7,
                        line_opacity=0.2,
                        legend_name='Crime Rate in San Fransico')
  crime_map

Image of the expected output：