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Thread: Magnify plot vertically while keeping position of baseline constant

  1. #1
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    Default 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:

    Qt Code:
    1. mag = new QwtPlotMagnifier(par->spectrum_plot->canvas());
    2. mag->setAxisEnabled(QwtPlot::yLeft, true);
    3. mag->setAxisEnabled(QwtPlot::xBottom, false);
    4. 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

  2. #2
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    Default Re: Magnify plot vertically while keeping position of baseline constant

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

    Uwe

  3. The following user says thank you to Uwe for this useful post:

    mariposa (5th September 2010)

  4. #3
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    Default Re: Magnify plot vertically while keeping position of baseline constant

    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:

    Qt Code:
    1. void Magnifier::rescale(double factor)
    2. {
    3. factor = qwtAbs(factor);
    4. if ( factor == 1.0 || factor == 0.0 )
    5. return;
    6.  
    7. bool doReplot = false;
    8. QwtPlot* plt = plot();
    9.  
    10. const bool autoReplot = plt->autoReplot();
    11. plt->setAutoReplot(false);
    12.  
    13. for ( int axisId = 0; axisId < QwtPlot::axisCnt; axisId++ )
    14. {
    15. const QwtScaleDiv *scaleDiv = plt->axisScaleDiv(axisId);
    16. if ( isAxisEnabled(axisId) && scaleDiv->isValid() )
    17. {
    18.  
    19. plt->setAxisScale(axisId, scaleDiv->lowerBound() * factor , scaleDiv->upperBound() * factor);
    20. doReplot = true;
    21. }
    22. }
    23.  
    24. plt->setAutoReplot(autoReplot);
    25.  
    26. if ( doReplot )
    27. plt->replot();
    28. }
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