new_contents: """
  Trail Marker
  ------------
  The ``trail`` marker is like the ``line`` marker, but it allows properties of
  the line (such as thickness) to vary along the length of the line.
  This shows a simple example of the trail mark using stock prices.
  """
  import altair as alt
  from vega_datasets import data
  
  stocks = data.stocks.url
  
  alt.Chart(stocks).mark_trail().encode(
      x='date:T',
      y='price:Q',
      size='price:Q',
      color='symbol:N'
  )
  
subject: Add trail_marker example