title: Altair doesn't display charts when running a script from terminal?
body: <p>I am following this tutorial example on my Mac Pro Big Sur.</p>
  <pre><code>https://altair-viz.github.io/gallery/simple_bar_chart.html
  </code></pre>
  <p>vtest.py is below:</p>
  <pre><code>import altair as alt
  import pandas as pd
  
  source = pd.DataFrame({
      'a': ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'],
      'b': [28, 55, 43, 91, 81, 53, 19, 87, 52]
  })
  
  alt.Chart(source).mark_bar().encode(
      x='a',
      y='b'
  )
  </code></pre>
  <p>When I execute this on a terminal, it doesn't display anything, no visualization showing up, no error or warning message either.</p>
  <pre><code>% python vtest.py
  </code></pre>
  <p>Doesn't Altair work on Mac OS?</p>