title: How to draw Google Line Charts when some of the values are missing?
body: <p>I've found the following JavaScript code on <a href="http://code.google.com/apis/chart/interactive/docs/gallery/imagelinechart.html" rel="noreferrer">Google Chart Tools</a>:</p>
  
  <pre><code>  function drawVisualization() {
      var data = new google.visualization.DataTable();
      data.addColumn('string', 'Year');
      data.addColumn('number', 'Sales');
      data.addColumn('number', 'Expenses');
      data.addRows(4);
      data.setValue(0, 0, '2004');
      data.setValue(0, 1, 1000);
      data.setValue(0, 2, 400);
      data.setValue(1, 0, '2005');
      data.setValue(1, 1, 1170); // sales for 2005
      data.setValue(1, 2, 460);
      data.setValue(2, 0, '2006');
      data.setValue(2, 1, 860);
      data.setValue(2, 2, 580);
      data.setValue(3, 0, '2007');
      data.setValue(3, 1, 1030);
      data.setValue(3, 2, 540);
  
      var chart = new google.visualization.ImageLineChart(document.getElementById('visualization'));
      chart.draw(data, {width: 500, height: 250, min: 0});
    }
  </code></pre>
  
  <p>If I comment out the line of code setting the value for the 2005 sales, the <em>Sales</em> line will appear on the graph starting from 2006 and ending in 2007. I was expecting to see the <em>Sales</em> line from 2004 (at Y=1000) to 2006 (at Y=860) and from 2006 (at Y=860) to 2007 (at Y=1030).</p>
  
  <p>How do I draw that chart if I don't have the value for the 2005 sales, but I have the values for 2004, 2006 and 2007?</p>
  
  <p><strong>Actual result:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/gGSGF.png" alt="actual chart"></p>
  
  <p><strong>Expected result:</strong> (I added the value 930 for the 2005 sales only to show what I want to accomplish; I hope there's a better way of doing this without computing all the missing Y values for all the series)</p>
  
  <p><img src="https://i.stack.imgur.com/8ELrr.png" alt="expected chart"></p>