Thank you Uwe!

d) I found it (Stroustrup, page 223)

As work arround I get the pixmap now from the canvas cache. That works fine and I get a lot faster plot. May be there are better ways as you suggested in d) as we could disable the canvas cache. I will check that later.

Qt Code: 
  1. void Plot::drawItems(QPainter *pPainter, const QRect &pRect, const QwtScaleMap pMap[QwtPlot::axisCnt], const QwtPlotPrintFilter &pFilter) const
  2. {
  3.    QwtPlotItemList lAlwaysDrawnItems;
  4.    // null pixmap means cache not valid
  5.    bool lCacheValid = !mPixmapCache->isNull();
  6.  
  7.    const QwtPlotItemList& lItemList = itemList(); 
  8.    for (QwtPlotItemIterator lItemIter = lItemList.begin(); lItemIter != lItemList.end(); ++lItemIter)
  9.    {
  10.       QwtPlotItem *lItem = *lItemIter;
  11.       if (lItem && lItem->isVisible() )
  12.       {
  13.          if ( !(pFilter.options() & QwtPlotPrintFilter::PrintGrid) && lItem->rtti() == QwtPlotItem::Rtti_PlotGrid)
  14.          {
  15.             continue;
  16.          }
  17.          // render items below z-index 1000
  18.          // when cache is invalid only
  19.          if(lItem->z() < 1000)
  20.          {
  21.             if(!lCacheValid)
  22.             {
  23.                pPainter->save();
  24.                pPainter->setRenderHint(QPainter::Antialiasing, lItem->testRenderHint(QwtPlotItem::RenderAntialiased));
  25.                lItem->draw(pPainter, pMap[lItem->xAxis()], pMap[lItem->yAxis()], pRect);
  26.                pPainter->restore();           
  27.             }
  28.          }
  29.          else
  30.          {
  31.             // all items above z-index 1000 are added to a list in order to draw after cache content
  32.             lAlwaysDrawnItems.append(lItem);
  33.          }
  34.       }
  35.    }
  36.  
  37.    if(lCacheValid)
  38.    {
  39.       // if cache has been valid then paint from pixmap cache
  40.       pPainter->drawPixmap(pRect.topLeft(), *mPixmapCache);
  41.    }
  42.    else
  43.    {
  44.       mPixmapCache = new QPixmap(*canvas()->paintCache());
  45.    }
  46.  
  47.    // finally draw the rest to be rendered each time (z-index > 1000)
  48.    for (QwtPlotItemIterator lItemIter = lAlwaysDrawnItems.begin(); lItemIter != lAlwaysDrawnItems.end(); ++lItemIter)
  49.    {
  50.       QwtPlotItem *lItem = *lItemIter;
  51.       pPainter->save();
  52.       pPainter->setRenderHint(QPainter::Antialiasing, lItem->testRenderHint(QwtPlotItem::RenderAntialiased));
  53.       lItem->draw(pPainter, pMap[lItem->xAxis()], pMap[lItem->yAxis()], pRect);
  54.       pPainter->restore();      
  55.    }
  56. }
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Best Regards
Stefan