title: How to draw a background behind a plotLine label in a highstock chart
body: <p>I want to draw a background behind a plotLine label in a highstock chart.
  Using an example from the Highstock API, I came up with this code (crt is the chart object): </p>
  
  <pre><code>        var textbox = crt.yAxis[ 0 ].plotLinesAndBands[ 0 ].label;
          var box = textbox.getBBox();
          crt.renderer.rect(box.x - 3, box.y + 1, box.width + 6, box.height, 3).attr({
              fill: '#0c0',
              id: 'labelBack',
              opacity: .7,
              'stroke-width': 0,
              zIndex: 4
          }).add();
  </code></pre>
  
  <p>This draws a semi-transparent box behind the label as intended (the label has zIndex 5). However when the chart is resized, the box maintains the same position relative to the top-left of the chart, causing misalignment with the label text (the position of the label changes because of the chart resizing).
  I tried using the chart redraw event for this, but even though I can see that the event is fired, and the function is executed again, no other boxes are drawn (I was trying to get more boxes to appear on each redraw, planning to solve removing obsolete boxes in the next iteration).
  How can I solve this?</p>