Issue with QwtLogScaleEngine

[nmantelier]

Hello I have trouble with plot With LogScale.
My goals is to be able to switch between linearScale and LogScale for QwtPlot::xBottom direction

if (logarithmic)
    {
        m_plotWidget->setAxisScaleEngine(QwtPlot::xBottom, new QwtLogScaleEngine);
    }
    else
    {
        m_plotWidget->setAxisScaleEngine(QwtPlot::xBottom, new QwtLinearScaleEngine);
    }

if (logarithmic)
    {
        m_plotWidget->setAxisScaleEngine(QwtPlot::xBottom, new QwtLogScaleEngine);
    }
    else
    {
        m_plotWidget->setAxisScaleEngine(QwtPlot::xBottom, new QwtLinearScaleEngine);
    }

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But when trying to draw my curve starting by point (0,0)

in qwtToPolyLineFiltered()
points[0].rx() = round( xMap.transform( sample0.x() ) );
points[0].ry() = round( yMap.transform( sample0.y() ) );

Points is not correctly defined cause transform do not handle values out of bounds: (return d_p1 + ( log(0) - d_ts1 ) * d_cnv;
The starting point is completely wrong.

What i am doing wrong? and how could i do it works?

I am trying to change qwt source and in my case that works

inline double QwtScaleMap::transform( double s ) const
{
    if ( d_transform )
    {
        double boundValue = d_transform->bounded(s);
        if ( boundValue != s )
        {
            s = d_ts1;
        }
        else
        {   
            s = d_transform->transform( s );
        }
    }
 
    return d_p1 + ( s - d_ts1 ) * d_cnv;
}

inline double QwtScaleMap::transform( double s ) const
{
    if ( d_transform )
    {
        double boundValue = d_transform->bounded(s);
        if ( boundValue != s )
        {
            s = d_ts1;
        }
        else
        {   
            s = d_transform->transform( s );
        }
    }

    return d_p1 + ( s - d_ts1 ) * d_cnv;
}

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[Uwe]

logarithmic scales can never go to 0 - by definition.

Qwt could be smarter with error checks, but in the end it would slow down runtime ( well not that important as log is way more expensive ) without having an option to do something useful.
One way to get around it is to define a QwtTransform class that uses different transformations for different areas ( f.e linear or something based on pow for -1 <= x <= 1 ).

If you want to do something like in the posted code you don't need to patch Qwt. Simply derive from QwtTransform and implement your log with range checks in YourTransform::transform. The new transformation can be used by QwtScaleEngine::setTransformation.

You can get some inspiration from the scaleengine example.

Uwe

[nmantelier]

Thank Uwe for your quick answer.

I try that you explain, it looks nice but have to look deeper.

Have a nice weekend