Set semi-transparent background on QwtPlotZoomer RectRubberBand

[missoni]

Hello!
I guess this must be a pretty easy question, but I cannot answer it without your help. I want to draw a semi-transparent background on the zoom rectangle of the QwtPlotZoomer. I have tried setting QPen and QBrush, but this only changed the rubberband line style, not the interior of the selection rectangle.

My code as of now is:

PlotZoomer::PlotZoomer(int xAxis, int yAxis, QwtPlotCanvas* canvas) : QwtPlotZoomer(xAxis,yAxis,canvas)
{
	QBrush brush(Qt::darkBlue);
	QPen pen(brush,5,Qt::DotLine);
	setRubberBandPen(pen);

PlotZoomer::PlotZoomer(int xAxis, int yAxis, QwtPlotCanvas* canvas) : QwtPlotZoomer(xAxis,yAxis,canvas)
{
	QBrush brush(Qt::darkBlue);
	QPen pen(brush,5,Qt::DotLine);
	setRubberBandPen(pen);

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Thank you in advance!