Problem with zooming the plot

[junhonguk]

I implement the zooming functionalility for the plot.
However, when I selected an area, the zoomed plot comes out is not the same area as I selected. Usually it has less data points to be shown in the graph. I don't know where it went wrong.
To make my question more clear, please see the attached screenshots.

Any suggestion is welcome and appreciated.

zoom1.jpgzoom2.jpg

[sonulohani]

It doesnt matter if you have less points or more points. May be you've implemented wrong. See this for how to implement the zoomer. Or take the reference of Bode plot example.

class Zoomer: public QwtPlotZoomer
{
public:
    Zoomer(int xAxis, int yAxis, QwtPlotCanvas *canvas):
        QwtPlotZoomer(xAxis, yAxis, canvas)
    {
        setTrackerMode(QwtPicker::AlwaysOff);
        setRubberBand(QwtPicker::NoRubberBand);
 
        // RightButton: zoom out by 1
        // Ctrl+RightButton: zoom out to full size
 
        setMousePattern(QwtEventPattern::MouseSelect2,
            Qt::RightButton, Qt::ControlModifier);
        setMousePattern(QwtEventPattern::MouseSelect3,
            Qt::RightButton);
    }
};

class Zoomer: public QwtPlotZoomer
{
public:
    Zoomer(int xAxis, int yAxis, QwtPlotCanvas *canvas):
        QwtPlotZoomer(xAxis, yAxis, canvas)
    {
        setTrackerMode(QwtPicker::AlwaysOff);
        setRubberBand(QwtPicker::NoRubberBand);

        // RightButton: zoom out by 1
        // Ctrl+RightButton: zoom out to full size

        setMousePattern(QwtEventPattern::MouseSelect2,
            Qt::RightButton, Qt::ControlModifier);
        setMousePattern(QwtEventPattern::MouseSelect3,
            Qt::RightButton);
    }
};

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QwtPlotZoomer *d_zoomer[2];
 d_zoomer[0] = new Zoomer( QwtPlot::xBottom, QwtPlot::yLeft, 
        d_plot->canvas());   //here replace it with your qwtplot.
    d_zoomer[0]->setRubberBand(QwtPicker::RectRubberBand);
    d_zoomer[0]->setRubberBandPen(QColor(Qt::green));
    d_zoomer[0]->setTrackerMode(QwtPicker::ActiveOnly);
    d_zoomer[0]->setTrackerPen(QColor(Qt::white));
 
    d_zoomer[1] = new Zoomer(QwtPlot::xTop, QwtPlot::yRight,
         d_plot->canvas());    /*here replace it with your plot. Implement this only if you have x is at Top and y is at right position. By seeing the above image, this line is not required. Skip this. */

QwtPlotZoomer *d_zoomer[2];
 d_zoomer[0] = new Zoomer( QwtPlot::xBottom, QwtPlot::yLeft, 
        d_plot->canvas());   //here replace it with your qwtplot.
    d_zoomer[0]->setRubberBand(QwtPicker::RectRubberBand);
    d_zoomer[0]->setRubberBandPen(QColor(Qt::green));
    d_zoomer[0]->setTrackerMode(QwtPicker::ActiveOnly);
    d_zoomer[0]->setTrackerPen(QColor(Qt::white));

    d_zoomer[1] = new Zoomer(QwtPlot::xTop, QwtPlot::yRight,
         d_plot->canvas());    /*here replace it with your plot. Implement this only if you have x is at Top and y is at right position. By seeing the above image, this line is not required. Skip this. */

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I think you've understood.

[junhonguk]

You are right. I implemented it in a wrong way.
Initially I didn't subclass the QwtPlotZoomer class and just used it as it is. This probably is the reason why it behaves weird.
It works properly after used the code as you suggested.
Many thanks for your help.