title: Python Altair plot two standards of deviation
body: <p>Is there a way to make a Python Altair plot showing <em>two</em> standards of deviation? Using <code>mark_errorbar(extent='stdev')</code> shows just one standard deviation.</p>
  <pre><code># Only shows one standard deviation.
  alt.Chart(data).mark_errorbar(extent='stdev').encode(
          x=alt.X('quantity:O', title='Quantity'),
          y=alt.Y('value:Q', title='Value')
      )
  </code></pre>
  <p><a href="https://i.stack.imgur.com/ZFfF7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZFfF7.png" alt="enter image description here" /></a></p>