Strange lines in beginner plot...

[emigrand]

Hi,

first post, so please don't be too strict with me.

I'm biologist, not programmer, and it took me a while to get to this point. Now I'm stuck with a problem which I think is easy for a qwt-advanced. One of my final goals is to visualize data from an ad-converter with qwt. For getting accustomed with matters I tried to produce a simple plot with code I found somewhere (in a header?), but when I compile it, I find strange lines besides the curves I would expect:

class vtPlot : public QwtPlot
{
    Q_OBJECT
    static int const NPOINTS = 150;
    double x[NPOINTS], ysin[NPOINTS], ycos[NPOINTS];
 
public:
   explicit    vtPlot(QObject *parent = 0);
   void *myData();
   QwtPointArrayData *myArray[2];
};
 
vtPlot::vtPlot(QObject *parent)
{
}
 
void *vtPlot::myData()
{
    for(int i = 0; i < NPOINTS; i++)
    {
        x[i] = 0.1*i;
        ysin[i] = sin(x[i]);
        ycos[i] = cos(x[i]);
    }
    myArray[0] = new QwtPointArrayData(x,ysin,(sizeof(x)+sizeof(ysin)));     // actually not sure if the third argument is correct
    myArray[1] = new QwtPointArrayData(x,ycos,(sizeof(x)+sizeof(ycos)));
}
 
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
 
    vtPlot *myPlot = new vtPlot(&w);
    QwtPlotCurve *curve1 = new QwtPlotCurve("sinus");
    QwtPlotCurve *curve2 = new QwtPlotCurve("cosinus");
 
    myPlot->myData();
 
    curve1->setData(myPlot->myArray[0]);
    curve1->attach(myPlot);
 
    curve2->setData(myPlot->myArray[1]);
    curve2->attach(myPlot);
 
    myPlot->setTitle("Voltage [mV] - Time [s]");
    myPlot->setAxisTitle(myPlot->yLeft,"Voltage [mV]");
    myPlot->setAxisTitle(myPlot->xBottom,"Time [s]");
 
    w.setCentralWidget(myPlot);
 
    w.show();
 
    return a.exec();
}

class vtPlot : public QwtPlot
{
    Q_OBJECT
    static int const NPOINTS = 150;
    double x[NPOINTS], ysin[NPOINTS], ycos[NPOINTS];

public:
   explicit    vtPlot(QObject *parent = 0);
   void *myData();
   QwtPointArrayData *myArray[2];
};

vtPlot::vtPlot(QObject *parent)
{
}

void *vtPlot::myData()
{
    for(int i = 0; i < NPOINTS; i++)
    {
        x[i] = 0.1*i;
        ysin[i] = sin(x[i]);
        ycos[i] = cos(x[i]);
    }
    myArray[0] = new QwtPointArrayData(x,ysin,(sizeof(x)+sizeof(ysin)));     // actually not sure if the third argument is correct
    myArray[1] = new QwtPointArrayData(x,ycos,(sizeof(x)+sizeof(ycos)));
}


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

    vtPlot *myPlot = new vtPlot(&w);
    QwtPlotCurve *curve1 = new QwtPlotCurve("sinus");
    QwtPlotCurve *curve2 = new QwtPlotCurve("cosinus");

    myPlot->myData();

    curve1->setData(myPlot->myArray[0]);
    curve1->attach(myPlot);

    curve2->setData(myPlot->myArray[1]);
    curve2->attach(myPlot);

    myPlot->setTitle("Voltage [mV] - Time [s]");
    myPlot->setAxisTitle(myPlot->yLeft,"Voltage [mV]");
    myPlot->setAxisTitle(myPlot->xBottom,"Time [s]");

    w.setCentralWidget(myPlot);

    w.show();

    return a.exec();
}

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Gives me this:
graph.jpg

Of course I ensure having searched the forum and the www for help, but I wasn't successful.
Any hint would be appreciated.

Thanks a lot,
Emi