I found this solution


Qt Code:
  1. QPrinter printer(QPrinter::HighResolution);
  2. ...
  3. int pw = printer.pageRect().width();
  4. int ph =printer.pageRect().height();
  5.  
  6. p.begin(&printer);
  7.  
  8. p.save();
  9. m_document->adjustSize();
  10. QTransform tra;
  11. tra.scale(11,11);
  12. p.setTransform(tra);
  13.  
  14. m_document->drawContents(&p,printer.paperRect());
  15.  
  16.  
  17. p.restore();
  18. QRect rec(-10,0,pw-10,0.4*ph);
  19. p.translate(0,0.65*ph);
  20. solarDiagram->print(&p,rec,filter);
  21. p.end();
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This is good for A4 paper. The plot is rendered as a vector graphic and I like it.
But I would scale the painter automatically. The value "11" is taken from a trial-and-correct approach....