Change axis ticks and label to switch between millimeter and inch in QWT Plot

[bkausbk]

Hello everyone,

I want to switch QWT plot axis labels and ticks to match as best for displaying inch and millimeters without changing curve points. I created a custom QwtScaleDraw and overwrote label() method, but this will not change the ticks.

class QwtMyScaleDraw : public QwtScaleDraw {
public:
    QwtMyScaleDraw() : QwtScaleDraw() {
    }
protected:
    QwtText label(double value) const {
           // global conversion routine that creates a display string either as millimeter or inch
           return ConvertToString(value, ConversionFromMillimeter);
    }
};

class QwtMyScaleDraw : public QwtScaleDraw {
public:
    QwtMyScaleDraw() : QwtScaleDraw() {
    }
protected:
    QwtText label(double value) const {
           // global conversion routine that creates a display string either as millimeter or inch
           return ConvertToString(value, ConversionFromMillimeter);
    }
};

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You can see this behaviour on following picture:

QwtPlotScaleDemo.png

Scale on left is the original scale, scale in the middle is with my custom label and right is the desired scale.

What is the best approach to get my desired results without adding entire curves again with manually converted values?

[Uwe]

You have to derive from QwtLinearScaleEngine adding your inch/millimeter modes there.

I guess all what needs to be done is to overload divideScale/autoScale, where you convert the values into the current mode. Then you can simply call the method of the base class with the converted values. Of course the result of the base class needs to be reconverted to the incoming mode too.

Uwe

[bkausbk]

Thank you Uwe, I got it. In case others have a similar requirement, this is my code:

class QwtUnitScaleDraw : public QwtScaleDraw {
public:
	QwtUnitScaleDraw(double ConversionFactor = 1.0) : QwtScaleDraw(), _ConversionFactor(ConversionFactor) {
	}
protected:
	QwtText label(double value) const {
		return QwtScaleDraw::label(value * _ConversionFactor);
	}
protected:
	double _ConversionFactor;
};
 
class QwtUnitScaleEngine: public QwtLinearScaleEngine {
public:
	QwtUnitScaleEngine(double ConversionFactor = 1.0) : QwtLinearScaleEngine(10), _ConversionFactor(ConversionFactor) {
	}
 
	void autoScale(int maxSteps, double &x1, double &x2, double &stepSize) const {
		x1 *= _ConversionFactor;
		x2 *= _ConversionFactor;
		stepSize *= _ConversionFactor;
		QwtLinearScaleEngine::autoScale(maxSteps, x1, x2, stepSize);
		x1 /= _ConversionFactor;
		x2 /= _ConversionFactor;
		stepSize /= _ConversionFactor;
 
	}
 
	QwtScaleDiv divideScale(double x1, double x2, int numMajorSteps, int numMinorSteps, double stepSize = 0.0) const {
		x1 *= _ConversionFactor;
		x2 *= _ConversionFactor;
		stepSize *= _ConversionFactor;
 
		QwtScaleDiv Div = QwtLinearScaleEngine::divideScale(x1, x2, numMajorSteps, numMinorSteps, stepSize);
 
		QList<double> Ticks[QwtScaleDiv::NTickTypes];
 
		Ticks[QwtScaleDiv::MajorTick] = Div.ticks(QwtScaleDiv::MajorTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MajorTick].count(); i++) {
			Ticks[QwtScaleDiv::MajorTick][i] /= _ConversionFactor;
		}
		Ticks[QwtScaleDiv::MediumTick] = Div.ticks(QwtScaleDiv::MediumTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MediumTick].count(); i++) {
			Ticks[QwtScaleDiv::MediumTick][i] /= _ConversionFactor;
		}
		Ticks[QwtScaleDiv::MinorTick] = Div.ticks(QwtScaleDiv::MinorTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MinorTick].count(); i++) {
			Ticks[QwtScaleDiv::MinorTick][i] /= _ConversionFactor;
		}
 
		return QwtScaleDiv(QwtInterval(x1 / _ConversionFactor, x2 / _ConversionFactor), Ticks);
	}
protected:
	double _ConversionFactor;
};

class QwtUnitScaleDraw : public QwtScaleDraw {
public:
	QwtUnitScaleDraw(double ConversionFactor = 1.0) : QwtScaleDraw(), _ConversionFactor(ConversionFactor) {
	}
protected:
	QwtText label(double value) const {
		return QwtScaleDraw::label(value * _ConversionFactor);
	}
protected:
	double _ConversionFactor;
};

class QwtUnitScaleEngine: public QwtLinearScaleEngine {
public:
	QwtUnitScaleEngine(double ConversionFactor = 1.0) : QwtLinearScaleEngine(10), _ConversionFactor(ConversionFactor) {
	}

	void autoScale(int maxSteps, double &x1, double &x2, double &stepSize) const {
		x1 *= _ConversionFactor;
		x2 *= _ConversionFactor;
		stepSize *= _ConversionFactor;
		QwtLinearScaleEngine::autoScale(maxSteps, x1, x2, stepSize);
		x1 /= _ConversionFactor;
		x2 /= _ConversionFactor;
		stepSize /= _ConversionFactor;

	}

	QwtScaleDiv divideScale(double x1, double x2, int numMajorSteps, int numMinorSteps, double stepSize = 0.0) const {
		x1 *= _ConversionFactor;
		x2 *= _ConversionFactor;
		stepSize *= _ConversionFactor;

		QwtScaleDiv Div = QwtLinearScaleEngine::divideScale(x1, x2, numMajorSteps, numMinorSteps, stepSize);
		
		QList<double> Ticks[QwtScaleDiv::NTickTypes];

		Ticks[QwtScaleDiv::MajorTick] = Div.ticks(QwtScaleDiv::MajorTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MajorTick].count(); i++) {
			Ticks[QwtScaleDiv::MajorTick][i] /= _ConversionFactor;
		}
		Ticks[QwtScaleDiv::MediumTick] = Div.ticks(QwtScaleDiv::MediumTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MediumTick].count(); i++) {
			Ticks[QwtScaleDiv::MediumTick][i] /= _ConversionFactor;
		}
		Ticks[QwtScaleDiv::MinorTick] = Div.ticks(QwtScaleDiv::MinorTick);
		for (unsigned int i = 0; i < Ticks[QwtScaleDiv::MinorTick].count(); i++) {
			Ticks[QwtScaleDiv::MinorTick][i] /= _ConversionFactor;
		}

		return QwtScaleDiv(QwtInterval(x1 / _ConversionFactor, x2 / _ConversionFactor), Ticks);
	}
protected:
	double _ConversionFactor;
};

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