Hi,
I'm new to Qt/Qwt and I have a problem with the zoomer.
Within my program I use geographical coordinates to visualize some 'z' values and therefore I use QwtPlot, especially depending on scales sometimes zooming is necessary.

Here I use an adapted Zoomer class from the bode example:
Qt Code: 
  1. class MyZoomer: public QwtPlotZoomer
  2. {
  3. public:
  4.     MyZoomer(QwtPlotCanvas *canvas):
  5.         QwtPlotZoomer(canvas)
  6.     {
  7.         setTrackerMode(AlwaysOn);
  8.  
  9.     }
  10.  
  11.     virtual QwtText trackerText(const QwtDoublePoint &pos) const
  12.     {
  13.         QColor bg(Qt::white);
  14. #if QT_VERSION >= 0x040300
  15.         bg.setAlpha(200);
  16. #endif
  17.  
  18.         QwtText text = QwtPlotZoomer::trackerText(pos);
  19.         text.setBackgroundBrush( QBrush( bg ));
  20.         return text;
  21.     }
  22. };
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Zoom in and out works perfectly on the first loaded file.
My problem appears if I load a second file. The initial plot shows the correct "new" coordinate space, but if I try to zoom in it collapse and ends up in a region far off and the Picker show (0.000,0.000). I tried to find some postings with a similar problem and I found:
first
second

I think I've the same problem like kota, but the solutions to set a new zoomBase and/or to re-enable autoscaling doesn't work or I use it wrong.

This is my complete code fragment:
Qt Code: 
  1. ui->PlotInitial->detachItems();
  2.  
  3.     //** [Setup grid scheme]!
  4.     ui->PlotInitial->setCanvasBackground(QColor(Qt::white));
  5.     QwtPlotGrid *grid = new QwtPlotGrid;
  6.     grid->enableXMin(true);
  7.     grid->enableYMin(true);
  8.     grid->setMajPen(QPen(Qt::black, 0, Qt::DotLine));
  9.     grid->setMinPen(QPen(Qt::gray, 0 , Qt::DotLine));
  10.     grid->attach(ui->PlotInitial);
  11.     //** [Setup grid scheme]!
  12.  
  13.  
  14.     QString appPath = QApplication::applicationDirPath();
  15.     QString ColorMapFile = appPath + "/crp/topo.crp";
  16.     vector< vector<double> > ColorMatrix = make_2DimVector <double>(256, 4);
  17.     LoadFileIntoMatrix(ColorMapFile, ColorMatrix, 4, 256);
  18.  
  19.     QwtArray< double > xVal, yVal;
  20.  
  21.     double xmax = 0, ymax = 0, zmax = 0;
  22.     double xmin = 10000000000,  ymin = 10000000000, zmin = 10000000000;
  23.  
  24.     for (int i = 0; i < Rows;i ++) {
  25.        if (BigMatrix[i][0] < xmin)  xmin  = BigMatrix[i][0];
  26.        if (BigMatrix[i][0] > xmax) xmax = BigMatrix[i][0];
  27.        if (BigMatrix[i][1] < ymin)  ymin  = BigMatrix[i][1];
  28.        if (BigMatrix[i][1] > ymax) ymax = BigMatrix[i][1];
  29.  
  30.        if (BigMatrix[i][2] > zmax) zmax = BigMatrix[i][2];
  31.        if (BigMatrix[i][2] < zmin)  zmin  = BigMatrix[i][2];
  32.     }
  33.  
  34.      double zKorrektur = 0;
  35.      if (zmin < 0) {
  36.          zmax = zmax + abs(zmin);
  37.          zKorrektur = abs(zmin);
  38.      }
  39.  
  40.      for (int i = 0; i < Rows;i ++) {
  41.        xVal.push_back(BigMatrix[i][0]);
  42.        yVal.push_back(BigMatrix[i][1]);
  43.  
  44.        double zVal = BigMatrix[i][2] + zKorrektur;
  45.  
  46.        int ColorIndex = (zVal * 255) / zmax;
  47.        // Insert new curves
  48.        QwtSymbol symb;
  49.        symb.setStyle(QwtSymbol::Ellipse);
  50.  
  51.        QColor Color2Set = QColor::fromRgb(ColorMatrix[ColorIndex][1],ColorMatrix[ColorIndex][2],ColorMatrix[ColorIndex][3]);
  52.        symb.setBrush(Color2Set);
  53.        symb.setPen(Color2Set);
  54.        symb.setSize(5);
  55.  
  56.       QwtPlotCurve *cScatter = new QwtPlotCurve();
  57.       cScatter->setStyle(QwtPlotCurve::NoCurve);
  58.       cScatter->setSymbol(symb);
  59.       cScatter->setData(xVal,yVal);
  60.       cScatter->attach(ui->PlotInitial);
  61.       xVal.clear();
  62.       yVal.clear();
  63.     }
  64.  
  65.     //ui->PlotInitial->setAxisScale(QwtPlot::yLeft, ymin, ymax);
  66.     //ui->PlotInitial->setAxisScale(QwtPlot::xBottom, xmin, xmax);
  67.     //ui->PlotInitial->replot();
  68.  
  69.     ui->PlotInitial->setAxisAutoScale(QwtPlot::yLeft);
  70.     ui->PlotInitial->setAxisAutoScale(QwtPlot::xBottom);
  71.  
  72.  
  73.     //** [Zoomfunctions!]
  74.     zoomerI = new MyZoomer(ui->PlotInitial->canvas());
  75.     zoomerI->setZoomBase();
  76.  
  77.     #if QT_VERSION < 0x040000
  78.     zoomerI->setMousePattern(QwtEventPattern::MouseSelect2,
  79.         Qt::RightButton, Qt::ControlButton);
  80.     #else
  81.     zoomerI->setMousePattern(QwtEventPattern::MouseSelect2,
  82.         Qt::RightButton, Qt::ControlModifier);
  83.     #endif
  84.     zoomerI->setMousePattern(QwtEventPattern::MouseSelect3,
  85.         Qt::RightButton);
  86.     //** [Zoomfunctions!]
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I hope its not to confusing, but any help will be appreciated.

Thank you and kind regards!