title: Labels are not rendering for plotLines highcharts
body: <p>I'm trying to render plotLines on Highcharts. But somehow I'm not able to render labels on plotLines.</p>
  
  <p>Here is the code snippet:</p>
  
  <pre><code>var chart = new Highcharts.Chart({
          chart: {
              renderTo: 'view_content',
              type: 'line'
          },
          title: {
              text: 'Dummy Data by Region'
          },
          xAxis: {
              categories: ['Africa', 'America', 'Asia']
          },
          yAxis: {
              plotLines:[{
                  value:450,
                  color: '#ff0000',
                  width:2,
                  zIndex:4,
                  label:{text:'goal',verticalAlign: 'bottom',
              textAlign: 'right',}
              }]
          },
          series: [{
              name: 'Year 1800',
              data: [107, 31, 50]
          },
                  {
              name: 'Goal',
                      type: 'scatter',
                      marker: {
                  enabled: true
              },
              data: [450]
          }]
      });
  </code></pre>
  
  <p>And after chart is rendered I'm calling addPlotLines function.</p>
  
  <pre><code>chart.yAxis[0].addPlotLine({
          value: 35.5,
          color: 'green',
          width: 2,
          id: 'plot-line-1',
                      label:{text:"Testing"}
      });
  </code></pre>
  
  <p>Plotlines is getting rendered but label on it is not rendering. 
  <a href="https://i.stack.imgur.com/R1kxq.png" rel="nofollow noreferrer">Here is the screenshot:</a></p>
  
  <p>Am I missing anything here?</p>
  
  <p><strong>Jquery Version</strong>: 3.1.0</p>
  
  <p><strong>Highcharts Version</strong>: 6.0.3</p>