Thank you for your answer.

I made a solution just a bit more generic, which works well:
Qt Code: 
  1. class ScaleItem: public QwtPlotScaleItem
  2. {
  3. public:
  4.     ScaleItem(const QwtScaleDiv& scaleDiv, QwtScaleDraw::Alignment alignment = QwtScaleDraw::BottomScale,
  5.             const double pos = 0.0 ):
  6.         QwtPlotScaleItem( alignment, pos), m_scaleDiv(scaleDiv), m_alignment(alignment)
  7.     {
  8.     }
  9.  
  10. protected:
  11.     void updateScaleDiv( const QwtScaleDiv& xScaleDiv,
  12.         const QwtScaleDiv& yScaleDiv )
  13.     {
  14.         switch(m_alignment)
  15.         {
  16.         case QwtScaleDraw::BottomScale:
  17.         case QwtScaleDraw::TopScale:
  18.             m_scaleDiv.setInterval(xScaleDiv.interval());
  19.             QwtPlotScaleItem::updateScaleDiv( m_scaleDiv, yScaleDiv );
  20.             break;
  21.         case QwtScaleDraw::LeftScale:
  22.         case QwtScaleDraw::RightScale:
  23.             m_scaleDiv.setInterval(yScaleDiv.interval());
  24.             QwtPlotScaleItem::updateScaleDiv( xScaleDiv, m_scaleDiv);
  25.             break;
  26.         }
  27.  
  28.     }
  29. private:
  30.     QwtScaleDiv m_scaleDiv;
  31.     const QwtScaleDraw::Alignment m_alignment;
  32. };
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It gives the following expected result after zooming:
afterzoom.jpg

I know have a question about the labels of these scale. As you can see in the screenshot above, they are hardly reading because the overlap with the colored square. I am using QwtPlotScaleItem::scaleDraw()->setLabelAlignment(Qt::AlignBottom | Qt::AlignLeft); which improves the situation.
However i need to shift a bit the labels (on the left for the horizontal scale, and to the bottom for the vertical scale). I tried the QwtPlotScaleItem::scaleDraw()->setSpacing(40) but it does not shift in the direction I need. Do i need to subclass QwtScaleDraw for this ?

Again, many thanks !

Eric