Export QwtPlot to png file with help print

[maxomato]

Hi.

I try to export QwtPlot to png file. I use this function

void XTrendViewer::makeExport(  const QString& fileName )
{
    QPicture picture;
    QwtPlot::print( picture );
 
    QPixmap pixmap( 800, 600 );
    pixmap.fill();
 
    QPainter painter( &pixmap );
    painter.drawPicture( 0,0, picture );
    painter.end();
    pixmap.save( fileName );
}

void XTrendViewer::makeExport(  const QString& fileName )
{
    QPicture picture;
    QwtPlot::print( picture );

    QPixmap pixmap( 800, 600 );
    pixmap.fill();

    QPainter painter( &pixmap );
    painter.drawPicture( 0,0, picture );
    painter.end();
    pixmap.save( fileName );
}

To copy to clipboard, switch view to plain text mode 

But I am getting file with white space without image. How to export/save qwtPlot into png, jpg and etc.