Using logaritmic and linear axis scale together

[alperyazir]

Hey !

I am using qwt 6.1.0. I tried:

linEngine = new QwtLinearScaleEngine;

logEngine = new QwtLogScaleEngine;

these codes to make my axis logaritmic and linear. But I want use them together for examle;

1 - 10 ---> in this interval must be linear

10 - 10000 ---> in this interval must be logaritmic

1.jpg

is that possible?

[Uwe]

You can try something like this:

class MyTransform: public QwtTransform
{
public:
    MyTransform( double threshold ):
        m_threshold( threshold ),
        m_offset( threshold - log( threshold ) )
    {
    }
 
    virtual double transform( double value ) const
    {
        double v = qAbs( value );
        if ( v > m_threshold )
            v = m_offset + log( v );
 
        return value < 0.0 ? -v : v;
    }
 
    virtual double invTransform( double value ) const
    {
        double v = qAbs( value );
        if ( v > m_offset )
            v = qExp( v ) - m_offset;
 
        return value < 0.0 ? -v : v;
    }
 
    virtual double bounded( double value ) const
    {
        return value;
    }
 
    virtual QwtTransform *copy() const
    {
        return new MyTransform( m_threshold );
    }
 
private:
    const double m_threshold;
    const double m_offset;
};
 
...
QwtLinearScaleEngine *scaleEngine = new QwtLinearScaleEngine();
scaleEngine->setTransformation( new MyTransform( 10.0 ) );
plot->setAxisScaleEngine( QwtPlot::yLeft, scaleEngine );
 
QwtScaleDiv scaleDiv( -10000.0, 10000.0 );
 
QList<double> majorTicks;
majorTicks << -10000 << -1000 << -100 << -10 << -8 << -6 << -4 << -2 << 0 << 2 << 4 << 6 << 8 << 10 << 100 << 1000 << 10000;
scaleDiv.setTicks( QwtScaleDiv::MajorTick, majorTicks );
 
plot->setAxisScaleDiv( QwtPlot::yLeft, scaleDiv );

class MyTransform: public QwtTransform
{
public:
    MyTransform( double threshold ):
        m_threshold( threshold ),
        m_offset( threshold - log( threshold ) )
    {
    }

    virtual double transform( double value ) const
    {
        double v = qAbs( value );
        if ( v > m_threshold )
            v = m_offset + log( v );

        return value < 0.0 ? -v : v;
    }

    virtual double invTransform( double value ) const
    {
        double v = qAbs( value );
        if ( v > m_offset )
            v = qExp( v ) - m_offset;

        return value < 0.0 ? -v : v;
    }

    virtual double bounded( double value ) const
    {
        return value;
    }

    virtual QwtTransform *copy() const
    {
        return new MyTransform( m_threshold );
    }

private:
    const double m_threshold;
    const double m_offset;
};

...
QwtLinearScaleEngine *scaleEngine = new QwtLinearScaleEngine();
scaleEngine->setTransformation( new MyTransform( 10.0 ) );
plot->setAxisScaleEngine( QwtPlot::yLeft, scaleEngine );

QwtScaleDiv scaleDiv( -10000.0, 10000.0 );

QList<double> majorTicks;
majorTicks << -10000 << -1000 << -100 << -10 << -8 << -6 << -4 << -2 << 0 << 2 << 4 << 6 << 8 << 10 << 100 << 1000 << 10000;
scaleDiv.setTicks( QwtScaleDiv::MajorTick, majorTicks );

plot->setAxisScaleDiv( QwtPlot::yLeft, scaleDiv );

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If you can't set the ticks manually ( f.e. your application supports zooming ) you also have to implement your own type of QwtScaleEngine.

Uwe