How to know the value of a point on the graph?

[oddytz1989]

Hello. Can you please tell whether you can use, qwt To make the something like this:

tr3.jpg
Ie when you click on the chart, find the Y coordinate of the curve?

[FelixB]

no, you can't (without implementing that by yourself). a curve is basically a series of points. QwtPlotCurve can give you the nearest point to a specified coordinate.

If you want exactly the y-value for any x-value, you have to find the x coordinates x_left and x_right of the nearest points where your x0 is in the middle. the curve is a straight line between these two points. now you need the y-values y_left and y_right of these two points. Then you can calculate the y0 using some triangulation.

hth,
Felix

[oddytz1989]

Thanks, connected Qwtpicker:

d_picker = new QwtPlotPicker(QwtPlot::xBottom, QwtPlot::yLeft,
         QwtPlotPicker::CrossRubberBand, QwtPicker::AlwaysOn,
       myPlot->canvas());
   d_picker->setRubberBandPen(QColor(Qt::green));
   d_picker->setRubberBand(QwtPicker::CrossRubberBand);
   d_picker->setTrackerPen(QColor(Qt::white));
 
   int cp;
   cp = curv1->closestPoint(d_picker->trackerPosition());
   psetInterval->setNum(cp);
 
 
   connect(d_picker, SIGNAL(moved(QPoint)), SLOT(UpdVizir()));

d_picker = new QwtPlotPicker(QwtPlot::xBottom, QwtPlot::yLeft,
         QwtPlotPicker::CrossRubberBand, QwtPicker::AlwaysOn,
       myPlot->canvas());
   d_picker->setRubberBandPen(QColor(Qt::green));
   d_picker->setRubberBand(QwtPicker::CrossRubberBand);
   d_picker->setTrackerPen(QColor(Qt::white));

   int cp;
   cp = curv1->closestPoint(d_picker->trackerPosition());
   psetInterval->setNum(cp);


   connect(d_picker, SIGNAL(moved(QPoint)), SLOT(UpdVizir()));

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void TrendTop::UpdVizir()
{
    int cp;
    cp = curv1->closestPoint(d_picker->trackerPosition());
    psetInterval->setNum(cp);
}

void TrendTop::UpdVizir()
{
    int cp;
    cp = curv1->closestPoint(d_picker->trackerPosition());
    psetInterval->setNum(cp);
}

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I want to get the X coordinate in the label psetInterval. I tried all the signals picker, but psetInterval displayed an all-time, the number "1"
How can I fix it?

[Uwe]

QwtPicker::trackerPosition is a point in widget - not in plot coordinates.

Instead use a picker with a QwtPickerClickPointMachine and connect to "QwtPlotPicker::selected( const QPointF &pos )"
Here you get the clicked point translated in plot coordinates - not some widget coordinate that is used for displaying the tracker text !

Also note that QwtPlotCurve::closestPoint returns the index of a point - not a plot coordinate !

Uwe

[oddytz1989]

I corrected:

picker_m = new QwtPickerClickPointMachine();
picker_m->setState(QwtPickerMachine::PointSelection);
d_picker = new QwtPlotPicker(QwtPlot::xBottom, QwtPlot::yLeft,
         QwtPlotPicker::CrossRubberBand, QwtPicker::AlwaysOn,
       myPlot->canvas());
d_picker->setStateMachine(picker_m);
 
connect(d_picker, SIGNAL(selected(const QPointF &pos)), SLOT(UpdVizir()));

picker_m = new QwtPickerClickPointMachine();
picker_m->setState(QwtPickerMachine::PointSelection);
d_picker = new QwtPlotPicker(QwtPlot::xBottom, QwtPlot::yLeft,
         QwtPlotPicker::CrossRubberBand, QwtPicker::AlwaysOn,
       myPlot->canvas());
d_picker->setStateMachine(picker_m);

connect(d_picker, SIGNAL(selected(const QPointF &pos)), SLOT(UpdVizir()));

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But anyway, nothing has changed.

[Uwe]

Of course not - how should it work, when you throw away the coordinate of the selected signal and connect it to the same wrong UpdVizir slot.

Uwe

[oddytz1989]

Could you tell what I need to fix?

Object::connect: No such signal QwtPlotPicker::selected(const QPointF &pos)

Object::connect: No such signal QwtPlotPicker::selected(const QPointF &pos)

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void TrendTop::UpdVizir()
{
    int cp;
    cp = curv1->closestPoint(d_picker->trackerPosition());
    psetInterval->setNum(cp);
}

void TrendTop::UpdVizir()
{
    int cp;
    cp = curv1->closestPoint(d_picker->trackerPosition());
    psetInterval->setNum(cp);
}

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[Uwe]

Could you tell what I need to fix?

Sorry I wrote nonsense of course QwtPlotCurve::closestPoint want a point in widget coordinates. You have to write something like this:

connect( picker, SIGNAL( selected( const QPolygon & ), SLOT(UpdVizir( const QPolygon & )));

connect( picker, SIGNAL( selected( const QPolygon & ), SLOT(UpdVizir( const QPolygon & )));

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and

void TrendTop::UpdVizir( const QPolygon &points )
{
    if ( points.size() != 1 )
         return; // should never happen for point selections
 
    double dist;
    int index = curv1->closestPoint( points[0], &dist );
    if ( index >= 0 && dist <= ... )
    {
        const QPointF found = curv1->sample( index );
        ....
    }
}

void TrendTop::UpdVizir( const QPolygon &points )
{
    if ( points.size() != 1 )
         return; // should never happen for point selections

    double dist;
    int index = curv1->closestPoint( points[0], &dist );
    if ( index >= 0 && dist <= ... )
    {
        const QPointF found = curv1->sample( index );
        ....
    }
}

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[oddytz1989]

Biiiiig thanks. The last question. How on the chart when you click the mouse show a vertical line?. I wrote:

d_picker->setRubberBandPen(QColor(Qt::green));
   d_picker->setRubberBand(QwtPicker::VLineRubberBand);

d_picker->setRubberBandPen(QColor(Qt::green));
   d_picker->setRubberBand(QwtPicker::VLineRubberBand);

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But nothing works.

[Uwe]

Check the bode example. It uses a crosshair instead of a vertical line - but the rest should be the same.

Uwe