/*-----------------------InvertScaleTransformation-----------------------*/
class InvertScaleTransformation : public QwtScaleTransformation
{
public:
	InvertScaleTransformation() : QwtScaleTransformation(QwtScaleTransformation::Other) {}
	~InvertScaleTransformation() {}

	double xForm(double s, double s1, double s2, double p1, double p2) const
	{
		return p1 + (p2 - p1) / (s2 - s1) * (s2 - s - s1);
	}

	double invXForm(double p, double p1, double p2, double s1, double s2) const
	{
		return s2 - s1 + (s2 - s1) / (p2 - p1) * (p - p1);
	}

	QwtScaleTransformation *copy() const
	{
		return new InvertScaleTransformation();
	}
};

/*-----------------------InvertScaleEngine-----------------------*/
class InvertScaleEngine : public QwtLinearScaleEngine
{
public:
	InvertScaleEngine() : QwtLinearScaleEngine() {}
	~InvertScaleEngine() {}

	QwtScaleTransformation* transformation() const
	{
		return new InvertScaleTransformation();
	}
};
...
plot->setAxisScaleEngine(QwtPlot::yLeft, new InvertScaleEngine());