title: None values in pyqtgraph
body: <p>I am trying to make a graph from a data which includes null value (data from pandas) using pyqtgraph. </p>
  
  <p><a href="http://i.stack.imgur.com/F4poH.png" rel="noreferrer">pyqtgraph - yellow circle has null values</a></p>
  
  <p><a href="http://i.stack.imgur.com/FOIfh.png" rel="noreferrer">graph using excel</a></p>
  
  <p>i want the graph using pyqtgraph to be the same as the excel graph. in other words there will be no line in the region with null values.</p>