title: QDateTimeAxis() - series are not displayed
body: <p>I am building application which includes QCharts. Everything was working until I changed Value Axis to the DateTime axis. Now I don't see any series on the chart. I was trying methods which was provided in other topics on stack overflow but without success. </p>
  
  <p>I was trying as it was suggested in other topics to change datetime to msec since epoch when I am setting range of x axe - unfortunately with this method on x axe I see epoch time not current time.
  When I am setting range like now I see correct time on x axe but I don't see any series. </p>
  
  <p>I've checked series - there are correct points in the range of x, y axis.</p>
  
  <p>I am using python 3.7 and pyside2. </p>
  
  <pre><code>self.plot = QtCharts.QChart()
  self.add_series("Magnitude (Column 1)", [0, 1])
  self.chart_view = QtCharts.QChartView(self.plot)
  
  self.series = QtCharts.QLineSeries()
  self.series.setName(name)
  self.plot.addSeries(self.series)
  
  # Setting X-axis
  self.axis_x = QtCharts.QDateTimeAxis()
  self.axis_x.setTickCount(10)
  self.axis_x.setLabelsAngle(70)
  self.axis_x.setFormat("dd.MM.yy h:mm:ss")
  self.axis_x.setTitleText("Date")
  self.axis_x.setMax(QDateTime.currentDateTime().addSecs(60))
  self.axis_x.setMin(QDateTime.currentDateTime())
  
  # Setting Y-axis
  self.axis_y = QtCharts.QValueAxis()
  self.axis_y.setTickCount(7)
  self.axis_y.setLabelFormat("%i")
  self.axis_y.setTitleText("Temperature [celcious]")
  self.axis_y.setMax(30)
  self.axis_y.setMin(20)
  
  self.series.attachAxis(self.axis_x)
  self.series.attachAxis(self.axis_y)
  self.plot.addAxis(self.axis_x, Qt.AlignBottom)
  self.plot.addAxis(self.axis_y, Qt.AlignLeft)
  ...
  
  # Add points to the chart
  def addPoint(self):
      x = QDateTime.currentDateTime().toSecsSinceEpoch()
      y = float(20+self.i)
  
      self.series.append(x, y)
      print(self.series.points())
      self.i += 1
      print(QDateTime.currentDateTime().toMSecsSinceEpoch(),y)
  </code></pre>