Magnify plot vertically while keeping position of baseline constant

Hi,

I am using a QwtPlotMagnifier object. I disabled the xBottom-axis so that the plot is magnified in the vertical direction only. I set the mouse button to NoButton because I just want to zoom by using the mouse wheel. This is the corresponding code:

Code:

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mag = new QwtPlotMagnifier(par->spectrum_plot->canvas());
mag->setAxisEnabled(QwtPlot::yLeft, true);
mag->setAxisEnabled(QwtPlot::xBottom, false);
mag->setMouseButton(Qt::NoButton);

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Now this is my problem: The plot I want to magnify is a spectrum and I just want to increase the intensity of the signals by using the mouse wheel. But the baseline of my spectrum (y=0) usually is not centered with respect to the range of the y-axis because the signals are mostly positive and everything else was a waste of plot area.

Therefore, while magnifying, the baseline is gradually moving to the bottom part of the plot and eventually vanishes completely.

Now my question: Is there a way to magnify the y-axis while the y=0 position is held at the same screen position so that I keep my basline inside my plot?

Many thanks for any help :)

Overload QwtPlotMagnifier::rescale(). Looking into into its implementation will help with your code.

Uwe

Many thanks for your answer, Uwe. It helped to solve my problem :)

Just in case others have the same problem, here is the code for my reimplementation of the QwtPlotMagnifier::rescale function:

Code:

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void Magnifier::rescale(double factor)
{
    factor = qwtAbs(factor);
    if ( factor == 1.0 || factor == 0.0 )
        return;
 
    bool doReplot = false;
    QwtPlot* plt = plot();
 
    const bool autoReplot = plt->autoReplot();
    plt->setAutoReplot(false);
 
    for ( int axisId = 0; axisId < QwtPlot::axisCnt; axisId++ )
    {
        const QwtScaleDiv *scaleDiv = plt->axisScaleDiv(axisId);
        if ( isAxisEnabled(axisId) && scaleDiv->isValid() )
        {
 
            plt->setAxisScale(axisId, scaleDiv->lowerBound() * factor , scaleDiv->upperBound() * factor);
            doReplot = true;
        }
    }
 
    plt->setAutoReplot(autoReplot);
 
    if ( doReplot )
        plt->replot();
}

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