title: Set tickMinStep for altair Axis?
body: <p>I'm trying to set <code>tickMinStep</code> for the x-axis in <code>altair</code> so that the tick marks occur every two instead of every one.</p>
  
  <p>Here's the documentation where I found the <code>tickMinStep</code> parameter.
  <a href="https://altair-viz.github.io/user_guide/generated/core/altair.Axis.html" rel="nofollow noreferrer">https://altair-viz.github.io/user_guide/generated/core/altair.Axis.html</a></p>
  
  <p>This is the example code I'm working with:</p>
  
  <pre><code>import altair as alt
  from vega_datasets import data
  
  source = data.movies.url
  
  chart = alt.Chart(source).mark_bar().encode(
      alt.X('IMDB_Rating:Q', axis=alt.Axis(title='Rating')),
      alt.Y('count()',title="Number of Movies"),
  )
  </code></pre>
  
  <p>Both of these ways fail to set <code>tickMinStep</code>, and return <code>SchemaValidationError: Invalid specification</code>. Any ideas why these fail?</p>
  
  <p>1:</p>
  
  <pre><code>chart.configure_axisX(tickMinStep=2)
  </code></pre>
  
  <p>2:</p>
  
  <pre><code>chart = alt.Chart(source).mark_bar().encode(
      alt.X('IMDB_Rating:Q', axis=alt.Axis(title='Rating', tickMinStep=2)),
      alt.Y('count()',title="Number of Movies"),
  )
  </code></pre>