Plot x^2 curve

[qt_developer]

Hi all,

I have this code to plot the x^2 curve:

// add curves
    QwtPlotCurve *curve1 = new QwtPlotCurve("Graph 1");
 
    //set curve color
    curve1->setPen(QPen(Qt::red, 2));
 
    // add curves
    curve1->attach(ui->qwtPlotWidget);
 
    const int MAX_VALUES = 100;
    double x[2*MAX_VALUES], y1[2*MAX_VALUES];// x and y values
    for (int i (-MAX_VALUES); i < MAX_VALUES; i++)
    {
        x[i] = i;
        y1[i] = i*i;
    }
 
    // copy the data into the curves
    curve1->setSamples(x, y1, 2*MAX_VALUES);
 
    // finally, refresh the plot
    ui->qwtPlotWidget->replot();

// add curves
    QwtPlotCurve *curve1 = new QwtPlotCurve("Graph 1");

    //set curve color
    curve1->setPen(QPen(Qt::red, 2));

    // add curves
    curve1->attach(ui->qwtPlotWidget);

    const int MAX_VALUES = 100;
    double x[2*MAX_VALUES], y1[2*MAX_VALUES];// x and y values
    for (int i (-MAX_VALUES); i < MAX_VALUES; i++)
    {
        x[i] = i;
        y1[i] = i*i;
    }

    // copy the data into the curves
    curve1->setSamples(x, y1, 2*MAX_VALUES);

    // finally, refresh the plot
    ui->qwtPlotWidget->replot();

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But I get this strange plot:

What's wrong?

Regards.