Problem of using QwtLog10ScaleEngine with QwtPlotHistogram

[lunatik-210]

Version: qwt 6.0.1
I've tried to develop logarithmic scaling for Histogram (As it works fine for curve, actually).
But the error is that the data are not drawn, so the plot is empty.

I've used simple line to enable scaling plot->setAxisScaleEngine(QwtPlot::yLeft, new QwtLog10ScaleEngine);

#ifndef PLOT_H
#define PLOT_H
 
#include <qwt_plot.h>
#include <qwt_plot_histogram.h>
#include <qwt_plot_curve.h>
 
 
class Plot : public QwtPlot
{
    public:
        Plot( QWidget *parent );
 
        void setData();
 
    private:
        QwtPlotHistogram* histogram;
 
};
#endif // PLOT_H

#ifndef PLOT_H
#define PLOT_H

#include <qwt_plot.h>
#include <qwt_plot_histogram.h>
#include <qwt_plot_curve.h>


class Plot : public QwtPlot
{
    public:
        Plot( QWidget *parent );

        void setData();

    private:
        QwtPlotHistogram* histogram;

};
#endif // PLOT_H

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#include "plot.h"
#include <qwt_scale_engine.h>
#include <qwt_interval.h>
 
Plot::Plot(QWidget *parent)
    : QwtPlot( parent )
{
    setAutoReplot( false );
 
    this->setAxisScaleEngine(QwtPlot::yLeft, new QwtLog10ScaleEngine);
    this->setAxisScale(QwtPlot::yLeft, 1.0, 100.0, 0.25);
    this->setAxisScale(QwtPlot::xBottom, 0.0, 100.0, 10.0);
 
    histogram = new QwtPlotHistogram("Histogramm");
    histogram->setPen( QPen( Qt::black ) );
    histogram->setYAxis( QwtPlot::yLeft );
    histogram->setXAxis( QwtPlot::xBottom );
    histogram->attach( this );
 
    setData();
 
    setAutoReplot( true );
 
}
 
void Plot::setData(){
    QVector<QwtIntervalSample> samples(4);
    samples.append(QwtIntervalSample(50.0, 0.0, 20.0));
    samples.append(QwtIntervalSample(80.0, 20.0, 40.0));
    samples.append(QwtIntervalSample(40.0, 40.0, 60.0));
    histogram->setData(new QwtIntervalSeriesData(samples));
}

#include "plot.h"
#include <qwt_scale_engine.h>
#include <qwt_interval.h>

Plot::Plot(QWidget *parent)
    : QwtPlot( parent )
{
    setAutoReplot( false );

    this->setAxisScaleEngine(QwtPlot::yLeft, new QwtLog10ScaleEngine);
    this->setAxisScale(QwtPlot::yLeft, 1.0, 100.0, 0.25);
    this->setAxisScale(QwtPlot::xBottom, 0.0, 100.0, 10.0);

    histogram = new QwtPlotHistogram("Histogramm");
    histogram->setPen( QPen( Qt::black ) );
    histogram->setYAxis( QwtPlot::yLeft );
    histogram->setXAxis( QwtPlot::xBottom );
    histogram->attach( this );

    setData();

    setAutoReplot( true );
    
}

void Plot::setData(){
    QVector<QwtIntervalSample> samples(4);
    samples.append(QwtIntervalSample(50.0, 0.0, 20.0));
    samples.append(QwtIntervalSample(80.0, 20.0, 40.0));
    samples.append(QwtIntervalSample(40.0, 40.0, 60.0));
    histogram->setData(new QwtIntervalSeriesData(samples));
}

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But in result I've an empty plot (see in attachments)! Could somebody help me, please!?

[Uwe]

In vertical direction the histogram draws bars from the baseline to the value for each sample. The default value for the baseline is 0.0, what is an invalid value for a logarithmic scale - that's why no bars are displayed at all.

After setting a valid baseline ( f.e. histogram->setBaseline( LOG_MIN ) ) you will see the missing bars.

Uwe

PS: The vector from your example code consists of 7 samples, where the first 4 are initialized with default ( also invalid ) values.

[lunatik-210]

Thank you, it works!!

ps Yeah, it my fault about vector size