title: In Highstock, overlapping flags when existing multiple series
body: <p>I have read <a href="https://stackoverflow.com/questions/23688459/highstock-two-flags-ond-the-same-xaxis-and-overlapping">https://stackoverflow.com/questions/23688459/highstock-two-flags-ond-the-same-xaxis-and-overlapping</a>. </p>
  
  <p>However,my chart has 5 series, after setting <em>stackDistance=40</em> and <em>onSeries: undefined</em> ,  the flags that have the same date still are overlapping, Is any solutions?</p>