qwt spectral plot stay zoomed after a replot()

[qt_gotcha]

My QwtPlot does not stay zoomed in or panned.

I have a spatial water model using QwtPlot to look at GIS type output while it is running. There are two spectralplots attached to it, one is semi transparent on top of another. Apologies for the long code!
The following is done once in a setup function

drawMap = new QwtPlotSpectrogram();
    drawMap->setRenderThreadCount( 0 );
    drawMap->attach( MPlot );
 
    baseMap = new QwtPlotSpectrogram();
    baseMap->setRenderThreadCount( 0 );
    baseMap->attach( MPlot );
 
    RD = new QwtMatrixRasterData();
    RDb = new QwtMatrixRasterData();
 
    rightAxis = new QwtScaleWidget();
/... etc/
 
    mapRescaler = new QwtPlotRescaler( MPlot->canvas() );
/... etc/
 
    magnifier = new QwtPlotMagnifier( MPlot->canvas() );
    magnifier->setAxisEnabled( MPlot->yRight, false );
 
    panner = new QwtPlotPanner( MPlot->canvas() );
    panner->setAxisEnabled( MPlot->yRight, false );

drawMap = new QwtPlotSpectrogram();
    drawMap->setRenderThreadCount( 0 );
    drawMap->attach( MPlot );

    baseMap = new QwtPlotSpectrogram();
    baseMap->setRenderThreadCount( 0 );
    baseMap->attach( MPlot );

    RD = new QwtMatrixRasterData();
    RDb = new QwtMatrixRasterData();

    rightAxis = new QwtScaleWidget();
/... etc/

    mapRescaler = new QwtPlotRescaler( MPlot->canvas() );
/... etc/

    magnifier = new QwtPlotMagnifier( MPlot->canvas() );
    magnifier->setAxisEnabled( MPlot->yRight, false );

    panner = new QwtPlotPanner( MPlot->canvas() );
    panner->setAxisEnabled( MPlot->yRight, false );

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then every model cycle I update the data in RD, I have a vector "mapdata" and replace the data in it and link it to the plot:
RD->setValueMatrix( mapData,nrCols );

Next I calculate the new maximum value and update the output and legend:. Because the user can select between 4 possible output maps, each with there own legend and colour scheme, these are updated also each timestep

if (op.drawMapType == 1)
    {
        MinV = 0.01;
        MPlot->setTitle("Runoff (l/s)");
        drawMap->setColorMap(new colorMapWaterLog());
        maxAxis1 = qMax(maxAxis1, MaxV);
        RD->setInterval( Qt::ZAxis, QwtInterval( 0, qMax(MinV, maxAxis1)));
    }
    else
        if (op.drawMapType == 2)
        { ... }
        else
            if (op.drawMapType == 3)
        { ... }
        else
                if (op.drawMapType == 4)
        { ... }
 
    drawMap->setData(RD);
    // link raster data to drawMap
 
    // add legend right of axis
    if (op.drawMapType == 1)
    {
        // log scale for runoff
        rightAxis->setColorMap( drawMap->data()->interval( Qt::ZAxis ), new colorMapWaterLog());
        if (maxAxis1 < 100)
            MPlot->setAxisScale( MPlot->yRight, MinV, qMax(1.0,maxAxis1));
        else
            MPlot->setAxisScale( MPlot->yRight, MinV, qMax(10.0,maxAxis1));
        MPlot->setAxisScaleEngine( MPlot->yRight, new QwtLog10ScaleEngine() );
    }
    else
        if (op.drawMapType == 2)
        { ... }
        else
            if (op.drawMapType == 3)
        { ... }
        else
                if (op.drawMapType == 4)
        { ... }
 
    MPlot->enableAxis( MPlot->yRight );
 
    MPlot->plotLayout()->setAlignCanvasToScales( true );
 
    MPlot->setAxisScale( MPlot->xBottom, 0.0, nrCols, nrCols/20);
    MPlot->setAxisMaxMinor( MPlot->xBottom, 0 );
    MPlot->setAxisScale( MPlot->yLeft, 0.0, nrRows, nrRows/20);
    MPlot->setAxisMaxMinor( MPlot->yLeft, 0 );
    mapRescaler->rescale();
 
    MPlot->replot(); //<=======

if (op.drawMapType == 1)
    {
        MinV = 0.01;
        MPlot->setTitle("Runoff (l/s)");
        drawMap->setColorMap(new colorMapWaterLog());
        maxAxis1 = qMax(maxAxis1, MaxV);
        RD->setInterval( Qt::ZAxis, QwtInterval( 0, qMax(MinV, maxAxis1)));
    }
    else
        if (op.drawMapType == 2)
        { ... }
        else
            if (op.drawMapType == 3)
        { ... }
        else
                if (op.drawMapType == 4)
        { ... }

    drawMap->setData(RD);
    // link raster data to drawMap

    // add legend right of axis
    if (op.drawMapType == 1)
    {
        // log scale for runoff
        rightAxis->setColorMap( drawMap->data()->interval( Qt::ZAxis ), new colorMapWaterLog());
        if (maxAxis1 < 100)
            MPlot->setAxisScale( MPlot->yRight, MinV, qMax(1.0,maxAxis1));
        else
            MPlot->setAxisScale( MPlot->yRight, MinV, qMax(10.0,maxAxis1));
        MPlot->setAxisScaleEngine( MPlot->yRight, new QwtLog10ScaleEngine() );
    }
    else
        if (op.drawMapType == 2)
        { ... }
        else
            if (op.drawMapType == 3)
        { ... }
        else
                if (op.drawMapType == 4)
        { ... }

    MPlot->enableAxis( MPlot->yRight );

    MPlot->plotLayout()->setAlignCanvasToScales( true );

    MPlot->setAxisScale( MPlot->xBottom, 0.0, nrCols, nrCols/20);
    MPlot->setAxisMaxMinor( MPlot->xBottom, 0 );
    MPlot->setAxisScale( MPlot->yLeft, 0.0, nrRows, nrRows/20);
    MPlot->setAxisMaxMinor( MPlot->yLeft, 0 );
    mapRescaler->rescale();

    MPlot->replot(); //<=======

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Evry cycle the plot resets to the original zoom level showing the whole plot, and does not stay zoomed in.
Obviously I am doing something wrong!

Thanks
Victor Jetten

P.S. Uwe: just a thought, have you considered microfinancing this thing, using kickstarter or something?

[Uwe]

MPlot->setAxisScale( MPlot->xBottom, 0.0, nrCols, nrCols/20);
MPlot->setAxisScale( MPlot->yLeft, 0.0, nrRows, nrRows/20);
MPlot->replot(); //<=======

MPlot->setAxisScale( MPlot->xBottom, 0.0, nrCols, nrCols/20);
MPlot->setAxisScale( MPlot->yLeft, 0.0, nrRows, nrRows/20);
MPlot->replot(); //<=======

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For your understanding: each scale has a current setting ( min/max + tick positions ) only and you are setting it explicitly with the lines above. Note that there is only 1 method changing the scale: QwtPlot::setAxisScaleDiv() - everything else ends up there.

"Zooming" also is nothing else than setting a new range, but QwtPlotZoomer maps user interactions to setAxisScale() calls + maintains a history of the previous scale ranges.

Your code has 2 effects:

1) you explicitly assign a new range
2) the zoom history is not aware of this new range and it won't appear in its history

How zooming works is unfortunately one of the issues Qwt users are confused most ( I had planned to come up with something more intuitive for Qwt 6.1, but was running out of time ). So I'm afraid you have to understand how the classes manipulating the scales play together - what is the first document I will write when Qwt 6,1 final is done.

have you considered microfinancing this thing, using kickstarter or something?

From time to time I receive donations ( details can be found at http://qwt.sf.net ), I'm using for buying stuff I need for Qwt development - like a Windows or VMWare license.

Uwe

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thanks, it works, very helpful