Thanks, Uwe.
I've improved my wonderful code:
Qt Code: 
  1. #include "qwt_plot.h"
  2. #include "qwt_scale_div.h"
  3. #include <QApplication>
  4.  
  5. int main(int argc, char ** argv) {
  6.   QApplication app(argc, argv);
  7.  
  8.   QwtPlot plot;
  9.   plot.setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
  10.   plot.setCanvasBackground(Qt::white);
  11.  
  12.  
  13.   QList<double> ticks[QwtScaleDiv::NTickTypes] = {
  14.     QList<double>(),
  15.     QList<double>(),
  16.     QList<double>()
  17.     << 10
  18.     << 40
  19.     << 90
  20.   };
  21.   QwtScaleDiv pressScaleDiv(0, 100, ticks);
  22.  
  23.   // QwtScaleDiv pressScaleDiv;
  24.   // pressScaleDiv.setInterval(0, 100);
  25.   // pressScaleDiv.setTicks(
  26.   //   QwtScaleDiv::MinorMajor,
  27.   //   QList<double>()
  28.   //   << 10
  29.   //   << 40
  30.   //   << 90
  31.   //   );
  32.   plot.setAxisScaleDiv(QwtPlot::yLeft, pressScaleDiv);
  33.  
  34.   plot.show();
  35.   return app.exec();
  36.  
  37. }
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and it works - but when I use the commented section instead of the working one above - it doesn't. No problem for me, anyway.
Thanks a lot.