title: Altair-viz: Is there a way to change the order that faceted charts get displayed?
body: <p>I want to facet my plot by column (straight-forward enough), but the faceted sub-plots are displayed in alphabetical order and I would like them displayed in a custom order. Using this example from the Altair website facets by species and displays the sub-plots in alphabetical order: 'setosa', 'versicolor', 'virginica'. </p>
  
  <pre><code>import altair as alt
  from altair.expr import datum
  from vega_datasets import data
  iris = data.iris.url
  
  alt.Chart(iris).mark_point().encode(
      x='petalLength:Q',
      y='petalWidth:Q',
      color='species:N',
      column='species:N'
  ).properties(
      width=180,
      height=180
  )
  </code></pre>
  
  <p>Is it possible to have this example display as: 'versicolor', 'virginica', 'setosa'? Thanks.</p>