title: QChart not displaying any series data when using QDateTimeAxis
body: <p>I want to display a real-time plot using <code>QChart</code> and <code>QLineSeries</code> where X values are timestamps. Everything works as expected when I use <code>QValueAxis</code> for X axis, but switching to <code>QDateTimeAxis</code> results in no data being plotted - just an empty chart.</p>
  
  <p>Sample code that demonstrates the problem:</p>
  
  <pre><code>QLineSeries *series = new QLineSeries();
  series-&gt;setUseOpenGL(true); // Disabling OpenGL doesn't change anything.
  
  QChart *chart = new QChart();
  chart-&gt;addSeries(series);
  
  QValueAxis *axisY = new QValueAxis();
  axisY-&gt;setTickCount(5);
  axisY-&gt;setMinorTickCount(1);
  axisY-&gt;setLabelFormat("%.2f");
  
  QDateTimeAxis *axisX = new QDateTimeAxis(); // Using QValueAxis here instead makes the problem disappear.
  axisX-&gt;setTitleText("Timestamp");
  axisX-&gt;setTickCount(5);
  
  chart-&gt;addAxis(axisX, Qt::AlignBottom);
  chart-&gt;addAxis(axisY, Qt::AlignLeft);
  series-&gt;attachAxis(axisX);
  series-&gt;attachAxis(axisY);  
  
  QChartView *chartView = new QChartView(chart);  
  chartView-&gt;setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform | QPainter::HighQualityAntialiasing);
  
  // Add some sample points.
  QDateTime xval;
  xval.setDate(QDate(2018, 1, 5));
  series-&gt;append(xval.toMSecsSinceEpoch(), 3);
  xval.setDate(QDate(2018, 1, 6));
  series-&gt;append(xval.toMSecsSinceEpoch(), 6);
  xval.setDate(QDate(2018, 1, 7));
  series-&gt;append(xval.toMSecsSinceEpoch(), 4);
  
  // Set ranges to display.
  chart-&gt;axisX()-&gt;setRange(series-&gt;at(0).x(), series-&gt;at(series-&gt;count()-1).x()); 
  chart-&gt;axisY()-&gt;setRange(0.0, 10.0);
  </code></pre>
  
  <p>This results in an empty chart with default X axis values ('01-01-1970 1:00' for all ticks), yet the same code renders the series correctly if <code>QValueAxis</code> is used instead of <code>QDateTimeAxis</code>.</p>
  
  <p>What I tried:</p>
  
  <ul>
  <li><code>chartView-&gt;repaint()</code> - has no effect;</li>
  <li><code>chart-&gt;removeSeries()/addSeries()</code> to re-add the series after appending the data; this causes the series to be displayed, but X axis tick values are wrong: they all show default '01-01-1970 ...' labels, not the ones corresponding to the data. Even if this were a working solution, it shouldn't be necessary to remove and re-add the series.</li>
  </ul>
  
  <p>I'm using Qt 5.9.2.</p>
  
  <p>Why does <code>QDateTimeAxis</code> behave differently? Is there a way to make this work consistently regardless of axis type?</p>