QwtPlotPicker::getInvTransform does not map to correct plot coordinates

[hakiim35]

Hello, I am implementing a curve widget in which the user can drag the curve by mouse move operations. The problem is the returned x dimension is not accurate in plot coordinates, there is no problem in the mapped y coordinate. There is always a shift or gap in the x dimension of the selected point(I guess this gap is about the width of the y-Axis ruler), please see the problem and the gap in my image attachment. Also I attach the code below.
I am using QwtPlotPicker::getInvTransform as,

class myCurveWidget : public QwtPlot
{
	/////////
}
 
class myCurvePicker : public QwtPlotPicker
{
	//////////
}
 
QPointF myCurveWidget::mapMousePoint(QPoint mousePoint)
{
	myCurvePicker picker(this->canvas());
 
	QPointF mappedPt = picker.getInvTransform(mousePoint);
 
	if(mappedPt.x() < 0)
		mappedPt.setX(0);
	else if(mappedPt.x() > 255)
		mappedPt.setX(255);
	if(mappedPt.y() < 0)
		mappedPt.setY(0);
	else if(mappedPt.y() > 255)
		mappedPt.setY(255);
 
	return mappedPt;
}

class myCurveWidget : public QwtPlot
{
	/////////
}

class myCurvePicker : public QwtPlotPicker
{
	//////////
}

QPointF myCurveWidget::mapMousePoint(QPoint mousePoint)
{
	myCurvePicker picker(this->canvas());
	
	QPointF mappedPt = picker.getInvTransform(mousePoint);
	
	if(mappedPt.x() < 0)
		mappedPt.setX(0);
	else if(mappedPt.x() > 255)
		mappedPt.setX(255);
	if(mappedPt.y() < 0)
		mappedPt.setY(0);
	else if(mappedPt.y() > 255)
		mappedPt.setY(255);
		
	return mappedPt;
}

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How can I map to exact plot coordinates?