title: NullPointerException when setting axis bounds on GraphView
body: <p>I am getting the following error:</p>
  
  <p><code>java.lang.NullPointerException: Attempt to invoke interface method 'double com.jjoe64.graphview.series.DataPointInterface.getX()' on a null object reference</code></p>
  
  <p>When I try to set the x and y axis bounds on my graph view.</p>
  
  <pre><code>    graph = (GraphView) findViewById(R.id.session_graph);
  
      graph.getViewport().setXAxisBoundsManual(true); // These lines seem to be causing it
      graph.getViewport().setMinX(1);
      graph.getViewport().setMaxX(50);
  
      graph.getViewport().setYAxisBoundsManual(true); // These lines seem to be causing it
      graph.getViewport().setMinY(2.0);
      graph.getViewport().setMaxY(15.0);
  </code></pre>
  
  <p>When I comment out the <code>setX/YBoundsManual()</code> lines, it runs without an error, but the bounds do not change, although the graph works fine. What could be causing this?</p>