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