title: Add legend to line & bars to Altair chart without using size/color
body: <p>I'm using Altair to create a chart with multiple lines each of which has multiple bands (representing different CI) &amp; I'm struggling to understand how I can add a legend. For example, in this considerably simpler example:</p>
  
  <pre><code>import altair as alt
  import pandas as pd
  
  df = pd.DataFrame(data={'col1': [1, 2,4,5,6], 'col2': [3, 4,7,4,4], 'col3': [1.5, 2.6,4.6,5.6,6.6], 'col4': [3.6, 4.6,7.6,4.6,4.4],'col5': [1.9, 2.9,4.9,5.9,6.9], 'col4': [3.9, 4.9,7.9,4.9,4.9]})
  
  
  line = alt.Chart(df).mark_line(color='purple').encode(
      x=alt.X('col1', title='Day'),
      y=alt.Y('col2', title='Column 2')
  )
  
  band_90 = alt.Chart(df).mark_area(opacity=0.3, color='purple').encode(
      x=alt.X('col1', title='Day'),
      y='col3',
      y2='col4',
  )
  
  band_50 = alt.Chart(df).mark_area(opacity=0.2, color='purple').encode(
      x=alt.X('col1', title='Day'),
      y='col4',
      y2='col5',
  )
  
  alt.layer(
      line+band_90+band_50
  ).save('chart.html')
  </code></pre>
  
  <p>How can I add a legend for the line &amp; the bands? I understand that the normal way to do it is via size &amp; color but the data source &amp; data I'm using makes me wanna do it this way if possible?</p>
  
  <p>(Please note - the bands admittedly look v. silly, it's completely fake data)</p>