Get the plot curve to which the mouse is pointing to

[StefanK]

I found a solution by implementing a custom PlotPicker and evaluating following method within the trackerText() method.

bool PlotPicker::curveSelected(const QPoint pMousePos, PlotCurve* &pPlotCurve, double &pYValue) const
{
    double lDist = 10e10;
    int lIndex = -1;
 
    // get nearest curve point by iterating over plot items, filtering for curves and calculating minimum distance
    const QwtPlotItemList& lPlotItemList = plot()->itemList();
    for ( QwtPlotItemIterator lPlotItemIterator = lPlotItemList.begin();
       lPlotItemIterator != lPlotItemList.end(); ++lPlotItemIterator )
    {
        if ( (*lPlotItemIterator)->rtti() == QwtPlotItem::Rtti_PlotCurve )
        {
            PlotCurve *lTmpCurve = (PlotCurve*)(*lPlotItemIterator);
 
            double lTmpDist;
            int lTmpIndex = lTmpCurve->closestPoint(pMousePos, &lTmpDist);
            if ( lTmpDist < lDist && lTmpIndex > -1)
            {
               pPlotCurve = lTmpCurve;
               lDist = lTmpDist;
               lIndex = lTmpIndex;
            } 
        }
    }
 
    // check if mouse position is in tolerance
    if ( pPlotCurve && lDist < _pix_tolerance_ )
    {
       pYValue = pPlotCurve->y(lIndex);
       return true;
    }
    pPlotCurve = 0;
    return false;
}
 
QwtText PlotPicker::trackerText(const QPoint &pMousePos) const
{
   QwtText lText;
   double lSelectedYValue = 0.0;
   PlotCurve* lSelectedCurve = 0;
   if(curveSelected(pMousePos, lSelectedCurve, lSelectedYValue)) {
      // ... the code creating the label at the cursor position
   }
   return lText;
}

bool PlotPicker::curveSelected(const QPoint pMousePos, PlotCurve* &pPlotCurve, double &pYValue) const
{
    double lDist = 10e10;
    int lIndex = -1;

    // get nearest curve point by iterating over plot items, filtering for curves and calculating minimum distance
    const QwtPlotItemList& lPlotItemList = plot()->itemList();
    for ( QwtPlotItemIterator lPlotItemIterator = lPlotItemList.begin();
       lPlotItemIterator != lPlotItemList.end(); ++lPlotItemIterator )
    {
        if ( (*lPlotItemIterator)->rtti() == QwtPlotItem::Rtti_PlotCurve )
        {
            PlotCurve *lTmpCurve = (PlotCurve*)(*lPlotItemIterator);
         
            double lTmpDist;
            int lTmpIndex = lTmpCurve->closestPoint(pMousePos, &lTmpDist);
            if ( lTmpDist < lDist && lTmpIndex > -1)
            {
               pPlotCurve = lTmpCurve;
               lDist = lTmpDist;
               lIndex = lTmpIndex;
            } 
        }
    }

    // check if mouse position is in tolerance
    if ( pPlotCurve && lDist < _pix_tolerance_ )
    {
       pYValue = pPlotCurve->y(lIndex);
       return true;
    }
    pPlotCurve = 0;
    return false;
}

QwtText PlotPicker::trackerText(const QPoint &pMousePos) const
{
   QwtText lText;
   double lSelectedYValue = 0.0;
   PlotCurve* lSelectedCurve = 0;
   if(curveSelected(pMousePos, lSelectedCurve, lSelectedYValue)) {
      // ... the code creating the label at the cursor position
   }
   return lText;
}

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In advance I implemented there also a vertical marker following the cursor position. By adding a horizontal marker we would get crosshair functionality...

PlotPicker::PlotPicker(QwtPlot *pPlot) :
   QwtPlotPicker(pPlot->canvas())
{
   // create vertical mouse marker
   mMouseMarker = new QwtPlotMarker();
   mMouseMarker->attach(pPlot);
   mMouseMarker->setVisible(false);
   mMouseMarker->setLineStyle(QwtPlotMarker::VLine);
   mMouseMarker->setLinePen(QPen(Qt::DotLine));
}
 
void PlotPicker::widgetMouseMoveEvent(QMouseEvent *pMouseEvent)
{
   QwtPlotPicker::widgetMouseMoveEvent(pMouseEvent);
   qreal lXPosition = invTransform(pMouseEvent->pos()).x();
   mMouseMarker->setXValue(lXPosition);
   mMouseMarker->setVisible(true);
   plot()->replot();
}
 
void PlotPicker::widgetLeaveEvent(QEvent *pEvent)
{
   QwtPlotPicker::widgetLeaveEvent(pEvent);
   mMouseMarker->setVisible(false);
   plot()->replot();
}

PlotPicker::PlotPicker(QwtPlot *pPlot) :
   QwtPlotPicker(pPlot->canvas())
{
   // create vertical mouse marker
   mMouseMarker = new QwtPlotMarker();
   mMouseMarker->attach(pPlot);
   mMouseMarker->setVisible(false);
   mMouseMarker->setLineStyle(QwtPlotMarker::VLine);
   mMouseMarker->setLinePen(QPen(Qt::DotLine));
}

void PlotPicker::widgetMouseMoveEvent(QMouseEvent *pMouseEvent)
{
   QwtPlotPicker::widgetMouseMoveEvent(pMouseEvent);
   qreal lXPosition = invTransform(pMouseEvent->pos()).x();
   mMouseMarker->setXValue(lXPosition);
   mMouseMarker->setVisible(true);
   plot()->replot();
}

void PlotPicker::widgetLeaveEvent(QEvent *pEvent)
{
   QwtPlotPicker::widgetLeaveEvent(pEvent);
   mMouseMarker->setVisible(false);
   plot()->replot();
}

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... and it works!

Regards
Stefan