get the point of a fitted curve

[giusepped]

Sorry to repost again my problem.
I have a fitted curve (the attribute of the curve is setted to Fitted).
I suppose that in this way Qwt compute for me the interpolated points.
well, if it so, I would use these interpolated points, in order to do other calculations.
I use this code

QwtSplineCurveFitter *t;
   QwtSpline sp;
   t = (QwtSplineCurveFitter*)dSolar[11]->curveFitter();
   sp= t->spline();
   qDebug()<<"draw,spline..."<<sp.points();

QwtSplineCurveFitter *t;
   QwtSpline sp;
   t = (QwtSplineCurveFitter*)dSolar[11]->curveFitter();
   sp= t->spline();
   qDebug()<<"draw,spline..."<<sp.points();

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dSolar[11] is simply a QwtPlotCurve.
Why I get an empty QPolygonF?
Helps appreciated