title: drawing two spline series with different number of values
body: <p>i have to draw two different spline series. i got  the values of the first line from a webservice, instead the second one is composed by two only values that means that is a straight line.
  the code is the following</p>
  
  <pre><code>var options = {
      chart: {
          renderTo: 'container'
      },
      yAxis: {
          labels: {
              formatter: function () {
                  return '€ ' + this.value;
              }
          }
      },
      xAxis: {
          categories: seriesMesi
      },
      series: [
          {
              name: 'scenario sfavorevole',
              color: '#FF3300',
              type: 'spline',
              data: [0, 12175],
              marker: {
                  fillColor: 'none',
                  lineColor: null
              }
          },
          {
              name: 'Valori del piano',
              color: 'blue',
              type: 'spline',
              marker: {
                  fillColor: 'none',
                  lineColor: null
              }
          }]
  };
  
  options.series[1].data = seriesValoriPiano;
  var chart = new Highcharts.Chart(options);
  </code></pre>
  
  <p>I can't represent the straight line: it has to occupy the whole x axis that means between the first and the last x values, but in my case it occupies only the range between the first and the second x values.
  How can i do to represent a straight line that occupies all the x values?</p>
  
  <p>thank you very much</p>