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.venv/Lib/site-packages/altair/examples/selection_layer_bar_month.py

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+"""
+Interactive Average
+===================
+The plot below uses an interval selection, which causes the chart to include an interactive brush
+(shown in grey). The brush selection parameterizes the red guideline, which visualizes the average
+value within the selected interval.
+"""
+# category: interactive charts
+import altair as alt
+from vega_datasets import data
+
+source = data.seattle_weather()
+brush = alt.selection(type='interval', encodings=['x'])
+
+bars = alt.Chart().mark_bar().encode(
+    x='month(date):O',
+    y='mean(precipitation):Q',
+    opacity=alt.condition(brush, alt.OpacityValue(1), alt.OpacityValue(0.7)),
+).add_selection(
+    brush
+)
+
+line = alt.Chart().mark_rule(color='firebrick').encode(
+    y='mean(precipitation):Q',
+    size=alt.SizeValue(3)
+).transform_filter(
+    brush
+)
+
+alt.layer(bars, line, data=source)