title: trellis layered plots in altair/Vega-Lite
body: <p>I would like to compare multiple conditions of an altair (ultimately vega-lite) layered plot. The perfect solution would be to facet/trellis the plot so I can see the different conditions side by side. Unfortunately I cannot figure out how to give the command to plot the different conditions.</p>
  
  <p>Here is my attempt to implement my idea based on the example for layered plots:
  (<a href="https://github.com/ellisonbg/altair/blob/master/altair/notebooks/07-LayeredCharts.ipynb" rel="nofollow noreferrer">https://github.com/ellisonbg/altair/blob/master/altair/notebooks/07-LayeredCharts.ipynb</a>)</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  data = pd.DataFrame({'x':np.random.rand(10), 'y':np.random.rand(10), 'z':['a', 'b']*5})
  
  chart = LayeredChart(data)
  chart += Chart().mark_line().encode(x='x:Q', y='y:Q', column='z:Q')
  chart += Chart().mark_point().encode(x='x:Q', y='y:Q', column='z:Q')    
  chart 
  </code></pre>
  
  <p>When compared with the example I added the column 'z' with the two conditions, and the two column statements in the Chart definitions.
  This solution generates seemingly good Vega-lite code,  but no plot.  Alternatively I tried "chart = LayeredChart(data).encode(column='z:Q')" but I then got the error 'LayeredChart' object has no attribute 'encode'</p>
  
  <p>I am wondering whether it is possible to facet (trellis) layered plots at all and whether it will be possible in future Vega-Lite releases.</p>
  
  <p>I am using jupyter with Anaconda </p>