This is my code :
Qt Code: 
  1. Plot::Plot(QWidget *parent):
  2.     QwtPlot(parent)
  3. {
  4.     tab_x[0] = 0;    tab_x[1] = 200;    tab_x[2] = 400;    tab_x[3] = 600;    tab_x[4] = 1600;	tab_x[5] = 2000;
  5.     tab_x[6] = 2400;	tab_x[7] = 2800;	tab_x[8] = 3200;	tab_x[9] = 3600;	tab_x[10] =     
  6.  
  7.     tab_y[0] = mygamma(tab_x[0],4096,1023,0.5);
  8.     tab_y[1] = mygamma(tab_x[1],4096,1023,0.5);
  9.     tab_y[2] = mygamma(tab_x[2],4096,1023,0.5);
  10.     tab_y[3] = mygamma(tab_x[3],4096,1023,0.5);
  11.     tab_y[4] = mygamma(tab_x[4],4096,1023,0.5);
  12.     tab_y[5] = mygamma(tab_x[5],4096,1023,0.    
  13.     tab_y[6] = mygamma(tab_x[6],4096,1023,0.5);
  14.     tab_y[7] = mygamma(tab_x[7],4096,1023,0.5);
  15.     tab_y[8] = mygamma(tab_x[8],4096,1023,0.5);
  16.     tab_y[9] = mygamma(tab_x[9],4096,1023,0.5);
  17.     tab_y[10] = mygamma(tab_x[10],4096,1023,0.5);
  18.  
  19.     int nb_points = sizeof(tab_x) / sizeof(tab_x[0]);
  20.     QPolygonF points,points2;
  21.     for ( int i = 0; i < nb_points ; i++ )
  22.     {
  23.         points += QPointF(tab_x[i], tab_y[i]);
  24.     }
  25.  
  26.     m_Curve = new QwtPlotCurve();
  27.     m_CurveFitter = new QwtSplineCurveFitter();
  28.  
  29.     m_Curve->setCurveAttribute(QwtPlotCurve::Fitted, true);	// Otherwise I have got Lines and not smooth curve the fitted curve is better for me
  30.     m_Curve->setStyle(QwtPlotCurve::Lines);
  31.  
  32.     m_CurveFitter->setFitMode(m_CurveFitter->ParametricSpline);
  33.     m_CurveFitter->setSplineSize(4096);                                           // from my orignal 10 points I want 4096 points			
  34.  
  35.     m_Curve->setCurveFitter(m_CurveFitter);			
  36.     m_Curve->setData(tab_x, tab_y, sizeof(tab_x) / sizeof(tab_x[0]));
  37.     m_Curve->attach(this);	
  38. }
  39.  
  40.  
  41.  
  42. QPolygonF Plot::getPoints(int nCurve)
  43. {	
  44. 	QPolygonF points,InterpolatedPoints;
  45. m_Curve;
  46. m_CurveFitter
  47.  
  48.     for ( int i = 0; i < m_Curve->dataSize(); i++ )
  49.     {		
  50.         points += QPointF(m_Curve->x(i), m_Curve->y(i));		
  51.     }
  52.  
  53.     InterpolatedPoints = curveFitter->fitCurve(points);
  54.  
  55.     return InterpolatedPoints;
  56. }
  57.  
  58.  
  59. double mygamma(double in,int nbpoints_in,int nbpoints_out,double gma)
  60. {
  61. 	double x = in / nbpoints_in;
  62.  
  63. 	return (pow(x,gma) * nbpoints_out );
  64. }
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So I have to setCurveAttribute(QwtPlotCurve::Fitted, FALSE ); ?? I that case my curve is totally not smooth...
I don't understand how Qwt can trace a curve, and one cannot access the points of that curve...

I do understand in my code that the Interpolated points that I retrieve are not the one traced on my graph ..the Fitted parameter should change this ...but I need it.