QCustomPlot Discussion and Comments

Here's my code,quite easy.I want to get the value of itemText->position->pixelPosition ();.But why customPlot->axisRect ()->width () equals zero?

resize (800, 800);
	QHBoxLayout* h_layout = new QHBoxLayout (this);
	QCustomPlot* customPlot = new QCustomPlot (this);

	customPlot->resize (600, 600);

	h_layout->addWidget (customPlot);

	QCPItemText* itemText = new QCPItemText (customPlot);

	itemText->setPositionAlignment (Qt::AlignBottom | Qt::AlignHCenter);

	itemText->setText ("Text Item Demo");
	itemText->position->setType (QCPItemPosition::ptAxisRectRatio);
	itemText->position->setCoords (0.5, 0.5); 
	QPointF pixelPos = itemText->position->pixelPosition ();
	qDebug () << pixelPos;//output :(0,0)
	qDebug () << customPlot->axisRect ()->width ();//output :0
	int w = customPlot->axisRect ()->width ();//w=0

I finally got the reason.What i need is customPlot->replot ();