title: Negative Fill color in AreaSpline Graph
body: <p>I want to fill area color above the spline in area spline chart.
  I have tried to set negative fill color but no result.
  Can anyone let me know how to fill area color above the spline rather then below?
  Any help will be appreciated.</p>
  
  <pre><code>$(function () {
          $('#container').highcharts({
              chart: {
                  type: 'areaspline'
              },
              title: {
                  text: 'Average fruit consumption during one week'
              },
              legend: {
                  layout: 'vertical',
                  align: 'left',
                  verticalAlign: 'top',
                  x: 150,
                  y: 100,
                  floating: true,
                  borderWidth: 1,
                  backgroundColor: '#FFFFFF'
              },
              xAxis: {
                  categories: [
                      'Monday',
                      'Tuesday',
                      'Wednesday',
                      'Thursday',
                      'Friday',
                      'Saturday',
                      'Sunday'
                  ],
                  plotBands: [{ // visualize the weekend
                      from: 4.5,
                      to: 6.5,
                      color: 'rgba(68, 170, 213, .2)',
                  }]
              },
              yAxis: {
                  title: {
                      text: 'Fruit units'
                  }
              },
              tooltip: {
                  shared: true,
                  valueSuffix: ' units'
              },
              credits: {
                  enabled: false
              },
              plotOptions: {
                  areaspline: {
                      fillOpacity: 0.5
                  }
              },
              series: [{
                  name: 'John',
                  data: [3, 4, 3, 5, 4, 10, 12],
                  negativeFillColor: '#000000'
              }, {
                  name: 'Jane',
                  data: [1, 3, 4, 3, 3, 5, 4],
                  negativeFillColor: '#000000'
              }]
          });
      })
  </code></pre>
  
  <p>;</p>
  
  <p>Thanks,
  M</p>