new_contents: """
  Calculating Percentage of Total
  -------------------------------
  This chart demonstrates how to use a window transform to display data values
  as a percentage of total values.
  """
  
  import altair as alt
  import pandas as pd
  
  activities = pd.DataFrame({'Activity': ['Sleeping', 'Eating', 'TV', 'Work', 'Exercise'],
                             'Time': [8, 2, 4, 8, 2]})
  
  alt.Chart(activities).mark_bar().encode(
      x='PercentOfTotal:Q',
      y='Activity:N'
  ).transform_window(
      window=[alt.WindowFieldDef(op='sum', field='Time', **{'as': 'TotalTime'})],
      frame=[None, None]
  ).transform_calculate(
      PercentOfTotal="datum.Time / datum.TotalTime * 100"
  )
  
subject: Add simple window transform example