ok, I solved it myself by "if" and "else if" conditions:

Qt Code: 
  1. class SimpleData: public QwtData
  2. {
  3.     // The x values depend on its index and the y values
  4.     // can be calculated from the corresponding x value.
  5.     // So we don't need to store the values.
  6.     // Such an implementation is slower because every point
  7.     // has to be recalculated for every replot, but it demonstrates how
  8.     // QwtData can be used.
  9.  
  10. public:
  11.     SimpleData( double (*b)(double), double rang1[2], double rang2[2], double step)
  12.     {
  13.     	constraintFunc = b;
  14.     	_step = step;
  15.     	rangeX[0]= rang1[0];	rangeX[1] = rang1[1];
  16.     	rangeY[0]= rang2[0];	rangeY[1] = rang2[1];
  17.     	range = (rang1[1] - rang1[0])/step;
  18.     	if(range<0){
  19.     		range = range *(-1);
  20.     	}
  21.     	d_size = size_t(range);
  22.     }
  23.  
  24.     virtual QwtData *copy() const
  25.     {
  26.         return new SimpleData(*this);
  27.     }
  28.  
  29.     virtual size_t size() const
  30.     {
  31.         return d_size;
  32.     }
  33.  
  34.     virtual double x(size_t i) const
  35.     {
  36.     	if(rangeX[0]<0){
  37.         return (-(range- i) + i)/ range;
  38.     	}
  39.         else
  40.         	return ((range + i) + i)/ range;
  41.     }
  42.  
  43.     virtual double y(size_t i) const
  44.     {
  45.     	if((*constraintFunc)(x(i)) > rangeY[1]){
  46.     		return rangeY[1];
  47.     	}
  48.     	else if((*constraintFunc)(x(i)) < rangeY[0]){
  49.     		return rangeY[0];
  50.     	}
  51.     	else return (*constraintFunc)(x(i));
  52.     }
  53. private:
  54.     size_t d_size;
  55.     double range;
  56.     double _step;
  57. 	double (*constraintFunc)(double);	// function pointer for constraint function
  58. 	double rangeX[2];
  59. 	double rangeY[2];
  60. };
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