new_contents: """
  Locations of US Airports
  ========================
  This is a layered geographic visualization that shows the positions of US
  airports on a background of US states.
  """
  # category: maps
  import altair as alt
  from vega_datasets import data
  
  airports = data.airports()
  airports_counts = airports[['state','latitude', 'longitude']]
  
  # Take the mean as the position of state
  agg = {'latitude': 'mean',
           'longitude': 'mean'
                    }
  airports_counts = airports.groupby(['state']).agg(agg) 
  agg = {'latitude': 'count',
        
                    }
  
  airports_counts['Number of Airports'] =  airports.groupby(['state']).agg(agg)
  
  states = alt.topo_feature(data.us_10m.url, feature='states')
  
  # US states background
  background = alt.Chart(states).mark_geoshape(
      fill='lightgray',
      stroke='white'
  ).properties(
      width=500,
      height=300
  ).project('albersUsa')
  
  # airport positions on background
  points = alt.Chart(airports_counts).mark_circle().encode(
      longitude='longitude:Q',
      latitude='latitude:Q',
      size='Number of Airports:Q',
      color=alt.value('steelblue')
  )
  
  background + points
subject: add example for points overlay on maps