class RealData: public QwtData 
{ 
private: 
    size_t d_size; 
 
public: 
    RealData(size_t size) : d_size(size) {} 
    virtual QwtData *copy() const { return new RealData(d_size); } 
    virtual size_t size() const { return d_size; } 
    virtual double x(size_t i) const { return (0.001 * i); } 
    virtual double y(size_t i) const { return 1/sqrt(x(i)); } 
}; 
******************************************************************* 
    const unsigned int nPoints = 10000; 
    cGraph->setData(RealData(nPoints));

setAxisScaleEngine(QwtPlot::xBottom, new QwtLog10ScaleEngine);