I resolved this using a different way to initialize, taken from from one of the samples:

Qt Code: 
  1. QList<double> ticks[QwtScaleDiv::NTickTypes];
  2. QList<double> &majorTicks = ticks[QwtScaleDiv::MajorTick];
  3. QList<double> &minorTicks = ticks[QwtScaleDiv::MinorTick];
  4.  
  5. for (int i = 0; i < (20 + 1); i++)
  6. {
  7. 	if (i % 5 == 0)
  8. 	{
  9. 		majorTicks.append(i * 2000);
  10. 	}
  11. 	else
  12. 	{
  13. 		minorTicks.append(i * 2000);
  14. 	}
  15. }
  16.  
  17. // mTimeScaleDiv is a AxisScaleDiv member
  18. mTimeScaleDiv = QwtScaleDiv(majorTicks.first(), majorTicks.last(), ticks);
  19. bool isValid = mTimeScaleDiv.isValid(); // For debug reasons
  20.  
  21. // Sets the updated plot
  22. setAxisScaleDiv(QwtPlot::xBottom, mTimeScaleDiv);
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