text: """
  Line Chart with Layered Aggregates
  ----------------------------------
  This example shows how to make a multi-series line chart of the daily closing
  stock prices for AAPL, AMZN, GOOG, IBM, and MSFT between 2000 and 2010, along
  with a layered rule showing the average values.
  """
  # category: line charts
  import altair as alt
  from vega_datasets import data
  
  source = data.stocks()
  
  base = alt.Chart(source).properties(width=550)
  
  line = base.mark_line().encode(x="date", y="price", color="symbol")
  
  rule = base.mark_rule().encode(
      y="average(price)", color="symbol", size=alt.value(2)
  )
  
  line + rule
  