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"""
Text over a Heatmap
-------------------
An example of a layered chart of text over a heatmap using the cars dataset.
"""
# category: tables
import altair as alt
from vega_datasets import data
source = data.cars()
# Configure common options. We specify the aggregation
# as a transform here so we can reuse it in both layers.
base = alt.Chart(source).transform_aggregate(
mean_horsepower='mean(Horsepower)',
groupby=['Origin', 'Cylinders']
).encode(
alt.X('Cylinders:O'),
alt.Y('Origin:O'),
)
# Configure heatmap
heatmap = base.mark_rect().encode(
color=alt.Color('mean_horsepower:Q',
scale=alt.Scale(scheme='viridis'),
legend=alt.Legend(title="Mean of Horsepower"),
)
)
# Configure text
text = base.mark_text(baseline='middle').encode(
text=alt.Text('mean_horsepower:Q', format=".0f"),
color=alt.condition(
alt.datum.mean_horsepower > 150,
alt.value('black'),
alt.value('white')
)
)
# Draw the chart
heatmap + text
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