Qwt move marker with mouse between interpolated points(data interpolation)

[Eligijus]

Hello,

Let's say i have curve with coordinates (x, y)(1, 1) (2, 1) (3, 1) and a marker. I want to get more x points for my marker to snap on when i move it with mouse. In other words i want my marker to snap to 1.1 , 1.2 , 1.3 x coordinates( now it only snaps to 1, 2 ,3).

Code snippet of how i move my marker:

marker->setLineStyle(QwtPlotMarker::VLine);
    marker->attach(tracePlot);
 
    picker = new QwtPlotPicker(tracePlot->xBottom, tracePlot->yLeft, QwtPicker::NoRubberBand,
QwtPicker::AlwaysOff, tracePlot->canvas());
    pickerMachine = new QwtPickerDragPointMachine();
    picker->setStateMachine(pickerMachine);
 
connect(picker, &QwtPicker::moved, this, &Widget::moveMarker);
 
void Widget::moveMarker(QPoint actualMousePosition)
{
    if(!flagZoom)
    {
        QwtText label;
        QString temp;
        int x = curve->closestPoint(actualMousePosition, NULL);
        QPointF position = curve->sample(x);
        marker->setValue(position);
        temp = QString::number(marker->xValue());
        label.setText(temp);
        marker->setLabel(label);
    }
    tracePlot->replot();
}

marker->setLineStyle(QwtPlotMarker::VLine);
    marker->attach(tracePlot);

    picker = new QwtPlotPicker(tracePlot->xBottom, tracePlot->yLeft, QwtPicker::NoRubberBand,
QwtPicker::AlwaysOff, tracePlot->canvas());
    pickerMachine = new QwtPickerDragPointMachine();
    picker->setStateMachine(pickerMachine);

connect(picker, &QwtPicker::moved, this, &Widget::moveMarker);

void Widget::moveMarker(QPoint actualMousePosition)
{
    if(!flagZoom)
    {
        QwtText label;
        QString temp;
        int x = curve->closestPoint(actualMousePosition, NULL);
        QPointF position = curve->sample(x);
        marker->setValue(position);
        temp = QString::number(marker->xValue());
        label.setText(temp);
        marker->setLabel(label);
    }
    tracePlot->replot();
}

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From what i know marker moves between curve vector indexes because

curve->closestPoint(actualMousePosition, NULL);

curve->closestPoint(actualMousePosition, NULL);

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returns index of my data vector. So even tho i could use a fitter and get my curve interpolated my marker still would not move between those interpolated points only indexes. Does qwt have data interpolation methods? Or is there a way to move my marker differently? What's the best approach here?

[Uwe]

Guess you are looking for something like what is implemented in the curvetracker example.

Using a marker for such a feature might be the wrong idea as it ends up in full replots, what might be an expansive operation, when having many points. Using a widget overlay ( like in the example ) avoids this problem.
Sometimes you need both - the overlay while moving and a plot item when it has its final position. Such a thing is implemented in the itemeditor example.

Uwe

[Eligijus]

Thanks for quick answer. I thought about something like bode(plot curve) example where a marker would get it's coordinates from x axis but issue that comes up is that i can't get y value of my curve(where vertical marker and curve crosses).