text: """
  Multi-Line Highlight
  ====================
  This multi-line chart uses an invisible Voronoi tessellation to handle mouseover to
  identify the nearest point and then highlight the line on which the point falls.
  It is adapted from the Vega-Lite example found at
  https://bl.ocks.org/amitkaps/fe4238e716db53930b2f1a70d3401701
  """
  # category: interactive charts
  import altair as alt
  from vega_datasets import data
  
  stocks = data.stocks.url
  
  highlight = alt.selection(type='single', on='mouseover',
                            fields=['symbol'], nearest=True)
  
  base = alt.Chart(stocks).encode(
      x='date:T',
      y='price:Q',
      color='symbol:N'
  )
  
  points = base.mark_circle().encode(
      opacity=alt.value(0)
  ).properties(
      selection=highlight,
      width=600
  )
  
  lines = base.mark_line().encode(
      size=alt.condition(~highlight, alt.value(1), alt.value(3))
  )
  
  points + lines
  