title: When I plot time on the x axis with altair it doesn't show year. Is it possible to fix this?
body: <p>I really enjoy using altair.  However, sometimes when I plot time, it doesn't show the year and just months.   I tried changing the dimensions and still can't get it unless the width is so large it makes the image less user friendly.</p>
  <pre><code>import altair as alt
  df_all['Date'] = pd.to_datetime(df_all['Date'])
  
  alt.Chart(df_all).mark_line().encode(
      x='Date:T',
      y='Values',
      color='Key',
  ).properties(height=500, width=600)
  </code></pre>
  <p>The data has these types:</p>
  <pre><code>Date      datetime64[ns]
  Key               object
  Values           float64
  </code></pre>
  <p>And looks like this:</p>
  <pre><code>Date,Key,Values
  2020-01-12,Leisure - Family,100.000000
  2020-01-12,Leisure - Non-Family,100.000000
  2020-01-12,Business,100.000000
  2020-01-19,Leisure - Family,101.989826
  2020-01-19,Business,95.664472
  </code></pre>
  <p>Output looks like this:</p>
  <p><a href="https://imgur.com/a/D9AlvLj" rel="nofollow noreferrer">https://imgur.com/a/D9AlvLj</a></p>
  <p>Thanks a lot in advance!</p>