Many thanks for your answer, Uwe. It helped to solve my problem

Just in case others have the same problem, here is the code for my reimplementation of the QwtPlotMagnifier::rescale function:

Qt Code: 
  1. void Magnifier::rescale(double factor)
  2. {
  3.     factor = qwtAbs(factor);
  4.     if ( factor == 1.0 || factor == 0.0 )
  5.         return;
  6.  
  7.     bool doReplot = false;
  8.     QwtPlot* plt = plot();
  9.  
  10.     const bool autoReplot = plt->autoReplot();
  11.     plt->setAutoReplot(false);
  12.  
  13.     for ( int axisId = 0; axisId < QwtPlot::axisCnt; axisId++ )
  14.     {
  15.         const QwtScaleDiv *scaleDiv = plt->axisScaleDiv(axisId);
  16.         if ( isAxisEnabled(axisId) && scaleDiv->isValid() )
  17.         {
  18.  
  19.             plt->setAxisScale(axisId, scaleDiv->lowerBound() * factor , scaleDiv->upperBound() * factor);
  20.             doReplot = true;
  21.         }
  22.     }
  23.  
  24.     plt->setAutoReplot(autoReplot);
  25.  
  26.     if ( doReplot )
  27.         plt->replot();
  28. }
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