[Solved]-High Charts Polar / Spider chart with off-graph y-axis labels-Highcharts

This demo shows how to find labels (SVG elements) in the chart object and create additional gridline using SVG Renderer: http://jsfiddle.net/kkulig/eg7p3r48/

events: {
  render: function() {
    var yAxis = this.yAxis[0],
        renderer = this.renderer,
      label = yAxis.ticks[0].label;

      renderer.path(['M', label.xy.x, label.xy.y, 'l', label.xy.x - xOffset - 30, 0]).attr({
        stroke: 'black',
        'stroke-width': 1
      }).add();

  }
}

API reference: https://api.highcharts.com/class-reference/Highcharts.SVGRenderer#path