title: pyqtgraph plot widget is not update
body: <p>I have two working stream.<br>
  1 - receiving data from com port and sends signal. </p>
  
  <pre><code>def __packet_parser(self, *args):
          while self.__thred_is_runing:
              data = self.__connection.read_all()
              if data != b'':
                  self.change_data.emit(self.__readline())
                  self.__callback(self.__readline()) 
  </code></pre>
  
  <p>2 - draw graph.  </p>
  
  <pre><code>def set_value_by_plot_name(self, value, name='default'):
            self.__plots[name].setData(np.append(self.__plots[name].getData()[1][1:], value))  
  def __test(self, value):
          tmp = value.decode('windows-1252')
          data = tmp.split('\t')
          if len(data) == 10:
              self.__gr.set_value_by_plot_name(int(data[1]))
  def main(self):
          self.__window = Window()
          self.__gr = Graphics()
          w = self.__gr.get_widget()
          self.__window.add_widget(w)
          connect = COMPortConnection('COM7', 38400)
          connect.change_data.connect(self.__test)
          connect.open()
          self.__window.show()
  </code></pre>
  
  <p>few seconds everything works fine, and then ceases to be updated.
  What problem?
  Data is updated, picture is not.</p>