new_contents: """
  Interactive Legend
  ------------------
  The following shows how to create a chart with an interactive legend, by
  binding the selection to ``"legend"``. Such a binding only works with
  ``selection_single`` or ``selection_multi`` when projected over a single
  field or encoding.
  """
  # category: interactive charts
  import altair as alt
  from vega_datasets import data
  
  source = data.unemployment_across_industries.url
  
  selection = alt.selection_multi(fields=['series'], bind='legend')
  
  alt.Chart(source).mark_area().encode(
      alt.X('yearmonth(date):T', axis=alt.Axis(domain=False, format='%Y', tickSize=0)),
      alt.Y('sum(count):Q', stack='center', axis=None),
      alt.Color('series:N', scale=alt.Scale(scheme='category20b')),
      opacity=alt.condition(selection, alt.value(1), alt.value(0.2))
  ).add_selection(
      selection
  )
subject: add interactive legend example