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"""
Grouped Points with Proportional Symbols Map
============================================
This is a layered geographic visualization that groups points by state.
"""
# category: maps
import altair as alt
from vega_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).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']
).properties(
title='Number of airports in US'
)
background + points
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