title: Google Charts - interpolateNulls option not working for AnnotationChart
body: <p>I'm a beginner with Google Visualizations.  I'm having trouble with getting the "interpolateNulls: true" option working correctly with an AnnotationChart.  If I use the same datasets with a basic LineChart, it works as expected.  Here is my code and resulting charts:</p>
  
  <pre><code>    &lt;script type='text/javascript'&gt;
    google.load('visualization', '1.1', {'packages':['Annotationchart']});
    google.setOnLoadCallback(drawChart);
    function drawChart() {
      var data1 = new google.visualization.DataTable(&lt;?=$json1?&gt;);
      var data2 = new google.visualization.DataTable(&lt;?=$json2?&gt;);
  
      var joinedData = google.visualization.data.join(data1, data2, 'full', [[0, 0]], [1], [1]);
  
      var chart1 = new google.visualization.AnnotationChart(document.getElementById('chart1'));
  
      var options = {
        displayAnnotations: false,
    thickness: 2,
    colors: ['red', 'blue'],
    interpolateNulls: true,
        curveType: 'function',
      };
  
      chart1.draw(joinedData, options);
    }
  </code></pre>
  
  <p></p>
  
  <p>produces:</p>
  
  <p><a href="http://imgur.com/r5sqtvd" rel="nofollow">http://imgur.com/r5sqtvd</a></p>
  
  <p>Whereas:</p>
  
  <pre><code>    &lt;script type='text/javascript'&gt;
    google.load('visualization', '1', {'packages':['corechart']});
    google.setOnLoadCallback(drawChart);
    function drawChart() {
      var data1 = new google.visualization.DataTable(&lt;?=$json1?&gt;);
      var data2 = new google.visualization.DataTable(&lt;?=$json2?&gt;);
  
      var joinedData = google.visualization.data.join(data1, data2, 'full', [[0, 0]], [1], [1]);
  
      var chart1 = new google.visualization.LineChart(document.getElementById('chart1'));
  
      var options = {
        displayAnnotations: false,
    thickness: 2,
    colors: ['red', 'blue'],
    interpolateNulls: true,
        curveType: 'function',
      };
  
      chart1.draw(joinedData, options);
    }
  </code></pre>
  
  <p></p>
  
  <p>produces:</p>
  
  <p><a href="http://imgur.com/qk79hob" rel="nofollow">http://imgur.com/qk79hob</a></p>
  
  <p>What am I missing?</p>
  
  <p>Thanks in Advance!</p>