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'''
US Population Pyramid Over Time
===============================
A population pyramid shows the distribution of age groups within a population.
It uses a slider widget that is bound to the year to visualize the age
distribution over time.
'''
# category: case studies
import altair as alt
from vega_datasets import data
source = data.population.url
slider = alt.binding_range(min=1850, max=2000, step=10)
select_year = alt.selection_point(name='year', fields=['year'],
bind=slider, value={'year': 2000})
base = alt.Chart(source).add_params(
select_year
).transform_filter(
select_year
).transform_calculate(
gender=alt.expr.if_(alt.datum.sex == 1, 'Male', 'Female')
).properties(
width=250
)
color_scale = alt.Scale(domain=['Male', 'Female'],
range=['#1f77b4', '#e377c2'])
left = base.transform_filter(
alt.datum.gender == 'Female'
).encode(
y=alt.Y('age:O', axis=None),
x=alt.X('sum(people):Q',
title='population',
sort=alt.SortOrder('descending')),
color=alt.Color('gender:N', scale=color_scale, legend=None)
).mark_bar().properties(title='Female')
middle = base.encode(
y=alt.Y('age:O', axis=None),
text=alt.Text('age:Q'),
).mark_text().properties(width=20)
right = base.transform_filter(
alt.datum.gender == 'Male'
).encode(
y=alt.Y('age:O', axis=None),
x=alt.X('sum(people):Q', title='population'),
color=alt.Color('gender:N', scale=color_scale, legend=None)
).mark_bar().properties(title='Male')
alt.concat(left, middle, right, spacing=5)
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