A question about coordinate transform

[lazy_learner]

I reimplement the mousePressEvent and use QwtPlot::invTransform to transform the coordinate.But it don't work as I expected.

void plot :: mousePressEvent(QMouseEvent *e)
{
    if(e->button() == Qt::LeftButton)
    {
        QPointF temp = e->posF();
        QMessageBox::information(this,tr("information!"),tr("the original x posation is %1 and y posation is %2").arg(temp.x()).arg(temp.y()),QMessageBox::Ok|QMessageBox::Default,QMessageBox::NoButton,QMessageBox::NoButton);
//picker->invTransform()
        temp.setX(this->invTransform(QwtPlot::xBottom,temp.x()));
        temp.setY(this->invTransform(QwtPlot::yLeft,temp.y()));
 
        QMessageBox::information(this,tr("information!"),tr("changed  x posation is %1 and y posation is %2").arg(temp.x()).arg(temp.y()),QMessageBox::Ok|QMessageBox::Default,QMessageBox::NoButton,QMessageBox::NoButton);
 
    }
 
}

void plot :: mousePressEvent(QMouseEvent *e)
{
    if(e->button() == Qt::LeftButton)
    {
        QPointF temp = e->posF();
        QMessageBox::information(this,tr("information!"),tr("the original x posation is %1 and y posation is %2").arg(temp.x()).arg(temp.y()),QMessageBox::Ok|QMessageBox::Default,QMessageBox::NoButton,QMessageBox::NoButton);
//picker->invTransform()
        temp.setX(this->invTransform(QwtPlot::xBottom,temp.x()));
        temp.setY(this->invTransform(QwtPlot::yLeft,temp.y()));

        QMessageBox::information(this,tr("information!"),tr("changed  x posation is %1 and y posation is %2").arg(temp.x()).arg(temp.y()),QMessageBox::Ok|QMessageBox::Default,QMessageBox::NoButton,QMessageBox::NoButton);

    }

}

To copy to clipboard, switch view to plain text mode 

well , it look just like 2013-05-23 07:00:57的屏幕截图.png2013-05-23 07:00:34的屏幕截图.png

but the real coordinate in the axis is is not x=688.05 and y=553.371