title: Altair: How to Change Color of Line in Line Chart?
body: <p>I have some time series data. I'm comparing 2 different series (predictions vs actual for a machine learning model). I have a line chart with both lines, but I can't figure out how to change the color of the lines, as all examples I see rely on organizing the data in a completely different way.</p>
  <p>This is what my code looks like.</p>
  <pre><code>source = df
  
  line1 = alt.Chart(source).mark_line().encode(
      x='Date', 
      y='y_preds'
  )
  
  line2 = alt.Chart(source).mark_line().encode(
      x='Date', 
      y='y_actual'
  )
  
  line1 + line2
  </code></pre>
  <p>How could I color one of the lines red without reorganizing all the data?</p>