title: How to set tick labels in python pyqtgraph PlotWidget
body: <p>I am trying to plot a time series of a few hours per day date without blank time between each day. It works fine if I use  pg.GraphicsWindow.The tick labels are 9:00  10:00 ..17:00. If I use pg.PlotWidget or pg.PlotWindow, Alt2 and Alt3 it does not work but the normla 0.1 0.2 ..is displayed. The code runs and the TimeAxisItem class is called but the x-axis tick labels do not change. I have a larger program with Qt.QMainWindow() and QtGui.QGridLayout() which does not accept pg.GraphicsWindow(). What am I missing??. How can you set tick labels in a PlotWidget?</p>
  <pre><code># -*- coding: utf-8 -*-
  ''' Setting x-axis labels for time series 
  '''
  
  import datetime as dt
  from pyqtgraph.Qt import QtGui, QtCore
  import numpy as np
  import pyqtgraph as pg
  
  class TimeAxisItem(pg.AxisItem):
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
          tr=np.arange('2016-06-10 09:00', '2016-06-10 18:00', dtype='datetime64[h]')        
          tnorm=(tr-tr[0])/(tr[-1]-tr[0])  #Map time to 0.0-1.0
          ttick=list()
          for i,t in enumerate(tr):
              tstr=np.datetime64(t).astype(dt.datetime)
              ttick.append(  (tnorm[i],  tstr.strftime(&quot;%H:%M&quot;))  )   
          self.setTicks([ttick])
  
  def main():
      app = QtGui.QApplication([])
      x=np.arange(0.0, 1.0, 0.02)
      y=np.sin(2*np.pi*x)
  
      #Alt 1
      win = pg.GraphicsWindow(title=&quot;Basic plotting&quot;)            
      plot=win.addPlot(title='Timed data', axisItems={'bottom': TimeAxisItem(orientation='bottom')})
      plot.plot(x,y)
  
      # Alt 2
      #win = pg.PlotWidget(title=&quot;Basic plotting&quot;)            
      #win.plot(title='Timed data', axisItems={'bottom': TimeAxisItem(orientation='bottom')})
      #win.plot(x,y)
      
      #Alt 3
      #win=pg.PlotWindow(title=&quot;Basic plotting&quot;)
      #win.plot(title='Timed data', axisItems={'bottom': TimeAxisItem(orientation='bottom')})
      #win.plot(x,y)
  
      win.show()
      app.exec_()
  
  if __name__ == '__main__':
      main()
  </code></pre>