title: How do you add a single text label to the max point in an Altair scatter plot?
body: <p>I'm trying to create a scatter plot in Altair and I would like to add a text label or annotation to the point with the maximum y.</p>
  
  <p>I've been able to add text_marks to all points, but I can't figure out how to label a specific point based on max(y)</p>
  
  <p>Here's some example data:</p>
  
  <pre><code>df = pd.DataFrame(columns=['date', 'daily', 'total'], 
           data=[['2019-08-01', 29, 102370],
                 ['2019-08-02', 18, 102388],
                 ['2019-08-03', 19, 102407],
                 ['2019-08-04', 13, 102420],
                 ['2019-08-05', 29, 102449],
                 ['2019-08-06', 49, 102498],
                 ['2019-08-07', 31, 102529],
                 ['2019-08-08', 39, 102568],
                 ['2019-08-09', 23, 102591],
                 ['2019-08-10', 17, 102608],
                 ['2019-08-11', 18, 102626],
                 ['2019-08-12', 38, 102664],
                 ['2019-08-13', 22, 102686]])
  </code></pre>
  
  <p>This is what I came up with so far, but it doesn't do what I want and I think I may have over-complicated it</p>
  
  <pre><code>chart = alt.Chart(
    data=df,
  ).mark_line(
      color='red'
  ).encode(
      alt.X('date:T', title=''),
      alt.Y('daily:Q', title='')
  )
  
  text = alt.Chart(df).mark_text().encode(
      x=alt.X('max(date):T'),
      y=alt.Y('max(daily):Q'),
      text=alt.Text('max(daily):Q')
  )
  
  
  (chart + text)
  </code></pre>