title: Series for tooltip only in highcharts
body: <p>I need to use several series in tooltip in a highchart chart. These series should be completely invisible except tooltip. I 've tried setting visible to false. However in this case, legends for that series are still visible though faded. If I state "ignoreHiddenSeries: true", hidden series are not there at all and I cannot use them at tooltip. Is there a way for this type of usage? Currently I am keeping those series in global javascript arrays outside the highchart's scope and using them in tooltip formatter. I prefer to keep those data in highchart as well.</p>
  
  <p>By the way setting <strong>showInLegend: false, visible: false</strong> also makes the series unusable in tooltip.  </p>