Logarithmic Y-Axis, no lables visible with low delta Y (ymax-ymin)

[HappyCoder]

Hello,

i'm using a logarithmic y-Axis like this:

class LogScaleDraw : public QwtScaleDraw
{
public:
    LogScaleDraw() : QwtScaleDraw(){
    }
 
    virtual QwtText label(double value) const
    {
        return QString::number(value, 'E', 3);
    }
};
 
        pyLogScaleEngine = new QwtLogScaleEngine;
        pyLogScaleDraw = new LogScaleDraw;
        setAxisScaleEngine( QwtPlot::yLeft, pyLogScaleEngine );
        setAxisScaleDraw( QwtPlot::yLeft, pyLogScaleDraw );
        setAxisMaxMajor( QwtPlot::yLeft, 15 );
        setAxisMaxMinor( QwtPlot::yLeft, 10 );

class LogScaleDraw : public QwtScaleDraw
{
public:
    LogScaleDraw() : QwtScaleDraw(){
    }

    virtual QwtText label(double value) const
    {
        return QString::number(value, 'E', 3);
    }
};

        pyLogScaleEngine = new QwtLogScaleEngine;
        pyLogScaleDraw = new LogScaleDraw;
        setAxisScaleEngine( QwtPlot::yLeft, pyLogScaleEngine );
        setAxisScaleDraw( QwtPlot::yLeft, pyLogScaleDraw );
        setAxisMaxMajor( QwtPlot::yLeft, 15 );
        setAxisMaxMinor( QwtPlot::yLeft, 10 );

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It is possible to load our data into a plot and sometimes (with low delta y) the
lables on the y-axis are not painted (or not visible). It seems to be independ
from the absolut value, e.g. even if i have a low delta y within a -5 decade of pressure
the lables are not visible.

Any ideas?
Thx

[Uwe]

Well, as long as we are not talking about deltas, that are below the significance of doubles, I don't see why it should not be possible to display a valid scale.
For logarithmic scales there is indeed a minimum of 1.0e-150 and a maximum of 1.0e+150 - hardcoded in QwtLogTransform.

What is the exact range, that leads to the bad scale in your case ?

Uwe

[HappyCoder]

Hello Uwe,

we get our data vom vacuum transmitter and the range is normaly between 2000 and 1E-10, and if
the data changes over time or the loaded data is e.g. from 995 to 1e-4, we don't have a problem.

But if the pressure value is nearly constant, means ymin=0.955 and ymax=0.956 for 1 minute e.g.,
we don't see a scale (right scale in attached picture).
If found in QwtLogScaleEngine, that it will be checked if the delta Y is lower that logBase and that happens
in our case. In qwt_scale_engine.cpp we jump into this, because we have less than on step i guess:

line 814 to 838 from qwt_scale_engine.cpp:

if ( interval.maxValue() / interval.minValue() < logBase )
    {
        // scale width is less than one step -> try to build a linear scale
 
        QwtLinearScaleEngine linearScaler;
        linearScaler.setAttributes( attributes() );
        linearScaler.setReference( reference() );
        linearScaler.setMargins( lowerMargin(), upperMargin() );
 
        linearScaler.autoScale( maxNumSteps, x1, x2, stepSize );
 
        QwtInterval linearInterval = QwtInterval( x1, x2 ).normalized();
        linearInterval = linearInterval.limited( LOG_MIN, LOG_MAX );
 
        if ( linearInterval.maxValue() / linearInterval.minValue() < logBase )
        {
            // the aligned scale is still less than one step
            if ( stepSize < 0.0 )
                stepSize = -qwtLog( logBase, qAbs( stepSize ) );
            else
                stepSize = qwtLog( logBase, stepSize );     // THIS IS CALLED FINALY
 
            return;
        }
    }

if ( interval.maxValue() / interval.minValue() < logBase )
    {
        // scale width is less than one step -> try to build a linear scale

        QwtLinearScaleEngine linearScaler;
        linearScaler.setAttributes( attributes() );
        linearScaler.setReference( reference() );
        linearScaler.setMargins( lowerMargin(), upperMargin() );

        linearScaler.autoScale( maxNumSteps, x1, x2, stepSize );

        QwtInterval linearInterval = QwtInterval( x1, x2 ).normalized();
        linearInterval = linearInterval.limited( LOG_MIN, LOG_MAX );

        if ( linearInterval.maxValue() / linearInterval.minValue() < logBase )
        {
            // the aligned scale is still less than one step
            if ( stepSize < 0.0 )
                stepSize = -qwtLog( logBase, qAbs( stepSize ) );
            else
                stepSize = qwtLog( logBase, stepSize );     // THIS IS CALLED FINALY

            return;
        }
    }

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The interessing point is, if i zoom in OR our with QwtMagnifier i can see the scale again.
Thx

[Uwe]

Which min/max values lead errors ?

Uwe

[HappyCoder]

Hello Uwe,

we made a subclass ob QwtLogScaleEngine and change some code

line 909 to 915:

if ( stepSize != 0.0 )
        {
            if ( stepSize < 0.0 )
                // stepSize = -qPow( logBase, -stepSize );  // NOT WORKING
                stepSize = 0; // WORKING
            else
                stepSize = qPow( logBase, stepSize );
        }

if ( stepSize != 0.0 )
        {
            if ( stepSize < 0.0 )
                // stepSize = -qPow( logBase, -stepSize );  // NOT WORKING
                stepSize = 0; // WORKING
            else
                stepSize = qPow( logBase, stepSize );
        }

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If the stepSize is negative, we set it to zero and everything is fine.

Is that maybe a general bug of Qwt?

Thx

[Uwe]

Again: please provide a min/max combination, that leads to errors ?

Uwe

[HappyCoder]

The ranges are in the picture in first post:
e.g from picture ymin=0.955, ymax= 0.956

Stefan

[Uwe]

I tried the following code with Qwt from SVN ( 6.1 branch ):

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_scale_engine.h>
 
int main( int argc, char **argv )
{
    QApplication a( argc, argv );
 
    QwtPlot plot;
    plot.setTitle( "Plot Demo" );
    plot.setCanvasBackground( Qt::white );
 
    plot.setAxisScaleEngine( QwtPlot::yLeft, new QwtLogScaleEngine );
    plot.setAxisMaxMajor( QwtPlot::yLeft, 15 );
    plot.setAxisMaxMinor( QwtPlot::yLeft, 10 );
    plot.setAxisScale( QwtPlot::yLeft, 0.955, 0.956 );
 
    plot.resize( 600, 400 );
    plot.show(); 
 
    return a.exec();
}

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_scale_engine.h>

int main( int argc, char **argv )
{
    QApplication a( argc, argv );
    
    QwtPlot plot;
    plot.setTitle( "Plot Demo" );
    plot.setCanvasBackground( Qt::white );
    
    plot.setAxisScaleEngine( QwtPlot::yLeft, new QwtLogScaleEngine );
    plot.setAxisMaxMajor( QwtPlot::yLeft, 15 );
    plot.setAxisMaxMinor( QwtPlot::yLeft, 10 );
    plot.setAxisScale( QwtPlot::yLeft, 0.955, 0.956 );
    
    plot.resize( 600, 400 );
    plot.show(); 
    
    return a.exec();
}

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At least [0.955, 0.956] does not seem to be a problem.

Uwe