Infinite filling area curve

[AidenBlake]

So, could you please help to do the same thing with QwtPlotMarker? My idea is in using pen with semitransparent color. How i can draw ending line with QwtPlotMarker?
I coded:

QwtPlotMarker *marker = new QwtPlotMarker();
QPen pen;
pen.setColor(QColor(150, 150, 150, 127));
marker->setLineStyle(QwtPlotMarker::HLine);

QwtPlotMarker *marker = new QwtPlotMarker();
QPen pen;
pen.setColor(QColor(150, 150, 150, 127));
marker->setLineStyle(QwtPlotMarker::HLine);

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So, now, if i'd want to draw infinite filling area, i could calculate pen width and use

pen.setWidthF(width);
marker->setLinePen(pen);
setYValue(y);

pen.setWidthF(width);
marker->setLinePen(pen);
setYValue(y);

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and what about ending line? help please