title: Google chart/Visualization api Interpolate Null is possible for some data points only?
body: <p>i have implemented a google chart api like as follows</p>
  
  <pre><code>function drawVisualization() {
    // Create and populate the data table.
    var data = new google.visualization.DataTable();
  
    data.addColumn('number', 'Year');
    data.addColumn({type: 'string', role: 'annotation'});
    data.addColumn('number', 'Val 1');
    data.addColumn('number', 'Val 2');
  
    data.addRows([
    [2008,' ', 20, 15],
    [2008.5,' ', null, null],
    [2009,' ', 25, null],
    [2009.5,' ', null, null],
    [2010,' ', 30,33],
    [2010.5,' ', null, null],
    [2011,' ', 23, 38],
    ]); 
  
    // Create and draw the visualization.
    var ac = new google.visualization.ComboChart(document.getElementById('visualization'));
    ac.draw(data, {
      title : 'Google Chart api',
      interpolateNulls: false,
      vAxis: {},
      hAxis: {
              ticks:[{v: 2008, f: '2008'},{v: 2009, f: '2009'},{v: 2010, f: '2010'},{v: 2011, f: '2011'}]
             },
  
      seriesType: "bars",
      series: {0: {type: "line"},1: {type: "line"}}
    });
  }
  </code></pre>
  
  <p><strong>​actual chart</strong></p>
  
  <p><img src="https://i.stack.imgur.com/NJk27.png" alt="actual chart"></p>
  
  <p>having tick marks that's why i added data points with decimal values (2008.5, 2009.5 ...)
  and i have data points for lines on year values like 2008, 2009, 2010 ... only. </p>
  
  <p>If i set InterpolateNulls : true, means it connects the data points otherwise every data points are not connected and displayed autonomous.</p>
  
  <p>so can i set interpolateNulls : true for some data points or some columns only in google chart api?</p>
  
  <p><strong>expected chart</strong></p>
  
  <p><img src="https://i.stack.imgur.com/BBVo8.png" alt="enter image description here"></p>