title: dynamic name in Altair alt.condition
body: <p>I am following this example to create a bar chart with conditional color formatting on negative values: </p>
  
  <pre><code>import altair as alt
  from vega_datasets import data
  
  source = data.us_employment()
  
  alt.Chart(source).mark_bar().encode(
    x="month:T",
    y="nonfarm_change:Q",
    color=alt.condition(
        alt.datum.nonfarm_change &gt; 0,
        alt.value("steelblue"),  # The positive color
        alt.value("orange")  # The negative color
    )
   ).properties(width=600)
  </code></pre>
  
  <p>How can I replace <code>alt.datum.nonfarm_change</code> with an expression accepting a dynamic variable name e.g. different columns of a dataframe?</p>