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

[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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