"""
Grouped Points with Proportional Symbols Map
============================================
This is a layered geographic visualization that groups points by state.
"""
# category: maps
import altair as alt
from altair.datasets import data

airports = data.airports.url
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')

# Airports grouped by state
points = alt.Chart(airports, title='Number of airports in US').transform_aggregate( 
    latitude='mean(latitude)',
    longitude='mean(longitude)',
    count='count()',
    groupby=['state']
).mark_circle().encode(
    longitude='longitude:Q',
    latitude='latitude:Q',
    size=alt.Size('count:Q').title('Number of Airports'),
    color=alt.value('steelblue'),
    tooltip=['state:N','count:Q']
)

background + points
