get the point of a fitted curve

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

Code:

---

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

---

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

Maybe, the splines created inside QwtPlotCurve are local.
So, I tried this one:

Code:

---

QwtSplineCurveFitter sp;
 
 
 
                QPolygonF interpolatedPoints;
                interpolatedPoints = sp.fitCurve(sunP[11]);
                qDebug()<<interpolatedPoints;

---

but the result is a long list of equal points.