title: How to add custom AxisItem to existing PlotWidget?
body: <p>I'm trying to add custom <em>AxisItem</em> in pyqtgraph to existing <em>PlotWidget</em> that was generated by Qt Designer. There is related topic <a href="https://stackoverflow.com/questions/18373714/can-i-replace-an-axisitem-on-a-qt-creator-promoted-pyqtgraph-widget">here</a>, but there is no exact answer with code example and I cannot comment, so I've created a new topic. </p>
  
  <p>This is my custom <em>AxisItem</em> (based on <a href="https://gist.github.com/friendzis/4e98ebe2cf29c0c2c232/" rel="noreferrer">this</a> code):</p>
  
  <pre><code>import pyqtgraph as pg
  import datetime
  
  def int2td(ts):
      return(datetime.timedelta(seconds=float(ts)/1e6))
  
  class TimeAxisItem(pg.AxisItem):
      def __init__(self, *args, **kwargs):
          super(TimeAxisItem, self).__init__(*args, **kwargs)
      def tickStrings(self, values, scale, spacing):
          return [int2dt(value).strftime("%H:%M:%S") for value in values]
  </code></pre>
  
  <p>This is my main <em>QtPlotter</em> class:</p>
  
  <pre><code>from pyqtgraph.Qt import QtGui
  from template_pyqt import Ui_Form # Ui_Form is generated by Qt Designer
  
  class QtPlotter:
      def __init__(self):
          self.app = QtGui.QApplication([])
          self.win = QtGui.QWidget()
          self.ui = Ui_Form()
          self.ui.setupUi(self.win)
          self.win.show()
  
          self.ui_plot = self.ui.plot 
          self.ui_plot.showGrid(x=True, y=True)
  </code></pre>
  
  <p>And then I'm trying to add my custom <em>AxisItem</em>:</p>
  
  <pre><code>self.ui_plot.getPlotItem().axes['bottom']['item'] = TimeAxisItem(orientation='bottom')
  </code></pre>
  
  <p>I have no errors, but this does not give any effect.</p>