Saving plot as image, JPG not working

[gib]

This is strange. The following code works fine for PNG, BMP and TIF formats, but if I choose the JPG format the image is all black, except for a hint of the frame.

int w = this->width();
int h = this->height();
QPixmap pixmap(w, h);
pixmap.fill(Qt::white); 
QwtPlotPrintFilter filter;
int options = QwtPlotPrintFilter::PrintAll;
options &= ~QwtPlotPrintFilter::PrintBackground;
options |= QwtPlotPrintFilter::PrintFrameWithScales;
filter.setOptions(options);
this->print(pixmap, filter);
QString fileName = "zzz.jpg";
if (fileName.isEmpty()) {
	return;
}
pixmap.save(fileName,0,-1);

int w = this->width();
int h = this->height();
QPixmap pixmap(w, h);
pixmap.fill(Qt::white); 
QwtPlotPrintFilter filter;
int options = QwtPlotPrintFilter::PrintAll;
options &= ~QwtPlotPrintFilter::PrintBackground;
options |= QwtPlotPrintFilter::PrintFrameWithScales;
filter.setOptions(options);
this->print(pixmap, filter);
QString fileName = "zzz.jpg";
if (fileName.isEmpty()) {
	return;
}
pixmap.save(fileName,0,-1);

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I invoke this by detecting rightButtonClick on a QwtPlot.
Is there something special about a JPG?