how to plot scatter graph?

Hello,

I would like to plot some kind of scatter graph on my QwtPlot to indicate feasible and infeasible regions on a spectrogramm, which is displayed on this QwtPlot, too. I have got some constraints or inequalities of type:
f(x1, x2) >= 0 , e.g. x2- x1^2 >=0 . Now, I have to calculate which coordinates of the plot violate the constraint and thus indicate it with a cross, for example. In forum, I have seen already that a scatter plot can be done by means of QwtPlotCurve, whereas one must set NOCURVE attribute to it:

Code:

---

_curve = new QwtPlotCurve();
 
_curve->setCurveStyle(QwtPlotCurve::NoCurve);
 
 
QwtSymbol symbol;
 
symbol.setStyle(QwtSymbol::XCross);
 
symbol.setSize(QSize(6, 6));
 
_curve->setSymbol(symbol);

---

But how should I set the data to this curve then? If I use the example "SimplePlot" where a curve gets its data from SimpleData class, then it iterates only through x, but I need to iterate both through x1 and x2.
My idea was to write IF and ELSE conditions within constraint function, which returns 0 or 1 in regards to satisfying the constraint, thus I would get then a data array with 0 and 1 and could display it as scatter graph above the spectrogram plot.

Please give me a hint, which data container to use or generally how to manage my plan.

Thank you

Code:

---

class SimpleData: public QwtData
{
    // The x values depend on its index and the y values
    // can be calculated from the corresponding x value. 
    // So we don´t need to store the values.
    // Such an implementation is slower because every point 
    // has to be recalculated for every replot, but it demonstrates how
    // QwtData can be used.
 
public:
    SimpleData(double(*y)(double), size_t size):
        d_size(size),
        d_y(y)
    {
    }
 
    virtual QwtData *copy() const
    {
        return new SimpleData(d_y, d_size);
    }
 
    virtual size_t size() const
    {
        return d_size;
    }
 
    virtual double x(size_t i) const
    {
        return 0.1 * i;
    }
 
    virtual double y(size_t i) const
    {
        return d_y(x(i));
    }
private:
    size_t d_size;
    double(*d_y)(double);
};

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