title: Remove over all line in plot
body: <p>I am using Highstock to display some sensor data. As the Highstock update 1.3.8 came out today I upgraded and found that there is a new line displayed by default on the plot:</p>
  
  <p><img src="https://i.stack.imgur.com/uVOC6.png" alt="enter image description here"></p>
  
  <p>This new line seems to connect the very first and very last data point.
  Is there a way to disable this new unwanted feature? I could not find something mentioned in the changelog or in the API documentation.</p>