Saving Qwtplot to image

I am using QwtPlotRenderer to save plot to file. I have also used QImage::grabWidget() to save plot to QPixmap.

But in both cases the resulting image is being :

As you can see ,the curve is not visible in result image whereas in myplot->show() function I am using full output. How can I solve this ?

Here is my code :

#include "QApplication"
#include<qwt_plot_layout.h>
#include<qwt_plot_curve.h>
#include<qwt_scale_draw.h>
#include<qwt_scale_widget.h>
#include<qwt_legend.h>
#include<qwt_plot_renderer.h>
class TimeScaleDraw:public QwtScaleDraw
{
public:
    TimeScaleDraw(const QTime & base)
        :baseTime(base)
    {
    }
virtual QwtText label(double v)const
{
    QTime upTime = baseTime.addSecs((int)v);
    return upTime.toString();
}
private:
    QTime baseTime;
};

int main(int argc,char * argv[])
{
    QApplication a(argc,argv);

    QwtPlot * myPlot = new QwtPlot(NULL);

    myPlot->setAxisScaleDraw(QwtPlot::xBottom,new TimeScaleDraw(QTime::currentTime()));
    myPlot->setAxisTitle(QwtPlot::xBottom,"Time");
    myPlot->setAxisLabelRotation(QwtPlot::xBottom,-50.0);
    myPlot->setAxisLabelAlignment(QwtPlot::xBottom,Qt::AlignLeft|Qt::AlignBottom);

    myPlot->setAxisTitle(QwtPlot::yLeft,"Speed");
    QwtPlotCurve * cur = new QwtPlotCurve("Speed");
    QwtPointSeriesData * data = new QwtPointSeriesData;
    QVector<QPointF>* samples=new QVector<QPointF>;
    for ( int i=0;i<60;i++)
    {
        samples->push_back(QPointF(i,i*i));
    }

    data->setSamples(*samples);
    cur->setData(data);
    cur->attach(myPlot);

    //myPlot->show();


    QPixmap pix = QPixmap::grabWidget(myPlot);
    std::cout<<pix.save("der.png","PNG")<<std::endl;
    QPixmap pixmap (400,400);
    QPainter * painter=new QPainter(&pixmap);

    QwtPlotRenderer rend;

    rend.render(myPlot,painter,myPlot->geometry());
    pixmap.save("Dene.jpg");
    return a.exec();
}

I found the solution by trial and error method.It solved my problem but I am not sure if it is the rightest solution.If anyone can explain reason I would be happy.

After attaching the curve to plot and before saving it to image I called myplot->replot() and the result become as I expected.