"""
Grouped Bar Chart with Error Bars
---------------------------------
This example shows a grouped bar chart with error bars.
"""
# category: bar charts
import altair as alt
from altair.datasets import data

source = data.barley()

bars = alt.Chart().mark_bar().encode(
    x='year:O',
    y=alt.Y('mean(yield):Q').title('Mean Yield'),
    color='year:N',
)

error_bars = alt.Chart().mark_errorbar(extent='ci').encode(
    x='year:O',
    y='yield:Q'
)

alt.layer(bars, error_bars, data=source).facet(
    column='site:N'
)
