title: How to create an average line for an irregular time graph?
body: <p>I'm building irregular time graphs with HighCharts that at the moment look like so:</p>
  
  <p><img src="https://i.stack.imgur.com/gPywV.png" alt="graph"></p>
  
  <p>And I'm wondering if it's possible to create an 'average' line for the three (or possibly more in future) lines.</p>
  
  <p>It would start following the blue line, then go closer to the green line mid-January, etc.</p>
  
  <p>At the moment the code I'm working with looks like:</p>
  
  <pre><code>$('#chart').highcharts({
    chart: { type: 'spline' },
    title: { text: '' },
    xAxis: { type: 'datetime' },
    yAxis: {
      title: { text: '' }
    }
    series: [{
      name: 'Line 1',
      data: [
        [Date.UTC(2014,0,16), 173.33],
        [Date.UTC(2014,0,23), 163.33],
        [Date.UTC(2014,0,30), 137.67],
        [Date.UTC(2014,1,6), 176.33],
        [Date.UTC(2014,1,13), 178.67],
        [Date.UTC(2014,1,27), 167.33],
      ],
      color: 'purple'
    },
    {
      name: 'Line 2',
      data: [
        [Date.UTC(2014,0,11), 156.33],
        [Date.UTC(2014,1,15), 167.67],
      ],
      color: 'green'
    },
    {
      name: 'Line 3',
      data: [
        [Date.UTC(2014,0,1), 135],
        [Date.UTC(2014,0,5), 146.33],
        [Date.UTC(2014,0,27), 146.75],
      ],
      color: 'blue'
    }]
  });
  </code></pre>