picker = new QwtPlotPicker(QwtPlot::xBottom,QwtPlot::yLeft,
QwtPicker::RectRubberBand,QwtPicker::AlwaysOn, myPlot->canvas());
QwtPickerDragRectMachine* test = new QwtPickerDragRectMachine();
test->setState(QwtPickerMachine::RectSelection);
picker->setStateMachine(test);

connect(picker, SIGNAL(selected(QRectF)),this,SLOT( click_on_graph(QRectF)));

void Mainwindow::click_on_graph(QRectF area)
{ int start;double dist;
int width=area.width();
int w=area.topLeft().x();
int h=area.bottomRight().x();
qDebug()<<"points"<<w<<h<<width;//picker->trackerPosition()
QPoint G=area.topLeft().toPoint();

start = curve->closestPoint(G,&dist);
qDebug()<<"index="<<start;
for(int i=0;i<width;i++)
{
QPointF graphintensity=this->curve->sample(start+(i));
qDebug()<<"x="<<graphintensity;
}
}

Hi all please suggest ,to find correct index to access curve data