score:11

Accepted answer

expanding on grzegorz's solution to include showing the marker:

  tooltip: {
    pointformatter: function() {
      var point = this;
      return '<span style="color:' + point.color + '">\u25cf</span> ' + point.series.name + ': <b>' + (point.y > 0 ? 'on' : 'off') + '</b><br/>';
    }
  }

http://jsfiddle.net/0u915g9b/1/

note that the default html is shown in the docs: http://api.highcharts.com/highcharts/series%3cline%3e.tooltip.pointformatter


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