Fill area above baseline and below the curve

[Uwe]

Setting brush and baseline to your curve is almost what you need - beside clipping away the rectangle above your baseline before the curve is filled. But unfortunately QwtPlotCurve::fillCurve() is not virtual.

So what you try do is something like this:

virtual void YourCurve::drawLines(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    int from, int to) const
{
   painter->save();
 
   painter->setClipRect(...);
 
   const QPen pen = this->pen();
   setPen(Qt::transparent);
 
   QwtPlotCurve::drawLines(painter, xMap, yMap, from, to);
 
   setPen(pen);
 
   painter->restore();
 
   const QBrush brush = this->brush();
   setBrush(Qt::NoBrush);
 
   QwtPlotCurve::drawLines(painter, xMap, yMap, from, to);
 
   setBrush(brush);
}

virtual void YourCurve::drawLines(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    int from, int to) const
{
   painter->save();

   painter->setClipRect(...);

   const QPen pen = this->pen();
   setPen(Qt::transparent);

   QwtPlotCurve::drawLines(painter, xMap, yMap, from, to);

   setPen(pen);

   painter->restore();

   const QBrush brush = this->brush();
   setBrush(Qt::NoBrush);

   QwtPlotCurve::drawLines(painter, xMap, yMap, from, to);
   
   setBrush(brush);
}

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Uwe