new_contents: """
  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 = bars.mark_rule().encode(
      x='year:O',
      y='ci0(yield):Q',
      y2='ci1(yield):Q',
      color = alt.value('black')
  )
  
  alt.layer(bars, error_bars, data=source).facet(
      column='site:N'
  )
  
subject: Create grouped_bar_chart_with_error_bars.py