格式Altair choropleth map颜色比例尺

import altair as alt
from vega_datasets import data

world_map = alt.topo_feature(data.world_110m.url, 'countries')

chart = alt.Chart(world_map).mark_geoshape().encode(
    color='population:Q',
)

chart = chart.properties(
    projection={'type': 'identity', 'reflectY': True},
    width=600,
    height=400
).transform_lookup(
    lookup='id',
    from_=alt.LookupData(data=source_data, key='id', fields=['population'])
).project('identity')

color_scale = alt.Scale(scheme='reds')

chart = chart.encode(
    color=alt.Color('population:Q', scale=color_scale, legend=alt.Legend(title='Population'))
)

chart.show()