QwtPlotMarker are not displayed on the QwtPlot

Hello,

I try to display markers above a spectrogram. The spectrogram data is visualized correctly, but I cannot see any markers on it. The code I use for visualization is below. Could you tell me, where the mistake is or why the markers are not visible on the plot? Thank you.

best regards
Code:
// this function draws the spectrogram of benchmark function and the constraint curves
void Plot::drawNewData(){
 
	this->detachItems();
 
    d_spectrogram = new QwtPlotSpectrogram();
 
	// load the corresponding struct with optimization problem
    //
		optProbList = _mf->getOptProblems();
		unsigned index=	_mf->objFunction;
		double (*f)(vector<double> coord) = optProbList[index]->benchmark;
		double rangex1[2] = {optProbList[index]->min[_mf->dim_indices[0]], optProbList[index]->max[_mf->dim_indices[0]]};
		double rangex2[2] = {optProbList[index]->min[_mf->dim_indices[1]], optProbList[index]->max[_mf->dim_indices[1]]};
		QwtDoubleInterval value_interval = optProbList[index]->valuesRange;
 
		// here the data for ploting is set from the function "SpectrogramData()", in which the data is generated
		//
		d_spectrogram->setData(SpectrogramData((*f), rangex1, rangex2,   _mf->dim_indices , _mf->fixed_values, optProbList[index]->dimensions, value_interval));
		d_spectrogram->attach(this);		// attach the data to the plot widget
 
		// set the colors for intensity values
		//
		 QwtLinearColorMap colorMap(Qt::darkCyan, Qt::red);
		 colorMap.addColorStop(0.25, Qt::cyan);
		 colorMap.addColorStop(0.5, Qt::green);
		 colorMap.addColorStop(0.75, Qt::yellow);
		 d_spectrogram->setColorMap(colorMap);
 
		 // A color bar on the right axis
		 //
		 QwtScaleWidget *rightAxis = axisWidget(QwtPlot::yRight);
		 rightAxis->setTitle("Intensity");
		 rightAxis->setColorBarEnabled(true);
		 rightAxis->setColorMap(d_spectrogram->data().range(), d_spectrogram->colorMap());
 
		 // set the scale of color bar
		 //
		 setAxisScale(QwtPlot::yRight,
			  d_spectrogram->data().range().minValue(),
			  d_spectrogram->data().range().maxValue() );
		  enableAxis(QwtPlot::yRight);
 
	   // define the steps and number of contour lines
	   //
		  QwtValueList contourLevels;
					  double stepsize;
					  if(value_interval.width()<2){
						  stepsize = 0.2;
					  }
					  if(value_interval.width()>=5 && value_interval.width() < 20){
						  stepsize = 1;
					  }
					  if(value_interval.width()>=20 && value_interval.width() < 50){
						  stepsize = 5;
					  }
					  if (value_interval.width() >= 50 && value_interval.width() < 1000){
						  stepsize = 50;
					  }
					  if ( value_interval.width() >= 1000){
						  stepsize = 10000;
					  }
		  for ( double level = value_interval.minValue(); level < value_interval.maxValue(); level += stepsize )
			  contourLevels += level;
		  d_spectrogram->setContourLevels(contourLevels);
 
		  plotLayout()->setAlignCanvasToScales(true);
 
	//+++++++++++++++++ plot constraints curves if the option "use constraints" is selected ++++++++++++++++++++++
 
	if(_mf->useConstraints){		// if the user has checked the radio button "use constraints" in initialization...
 
		QVector<int> equality = optProbList[index]->constraint_equality;
 
		if(_mf->allConstraints){	// if use all constraints
			int k=0;
		for( std::map<QString, double (*)( vector<double>  )>::iterator ii = optProbList[index]->constraints.begin(); ii!= optProbList[index]->constraints.end(); ++ii)
			   {
 
			double step_size = 0.5 ;
				for(double x1= optProbList[index]->min[_mf->dim_indices[0]] ; x1 < optProbList[index]->max[_mf->dim_indices[0]] ; x1=x1+ step_size){
					for(double x2= optProbList[index]->min[_mf->dim_indices[1]] ; x2 < optProbList[index]->max[_mf->dim_indices[1]] ; x2=x2+ step_size){
						vector<double> coords(3,0);
						coords = _mf->fixed_values;
						coords[_mf->dim_indices[0]] = x1;
						coords[_mf->dim_indices[1]] = x2;
						double result = (*ii).second(coords);	// compute the y value
						if(result==1){
							// create PlotMarkers to visualize the infeasible area
							QwtPlotMarker *mark = new QwtPlotMarker();
								QwtSymbol *symb = new QwtSymbol();
								symb->setBrush(QBrush(Qt::red, Qt::SolidPattern));
								symb->setStyle(QwtSymbol::Ellipse);
								symb->setSize(5,5);
								mark->setSymbol(*symb);
								mark->setXValue(x1);						// set the position of marker on the plot
								mark->setYValue(x2);
								//mark->setZ(101);
								mark->attach(this);
						}
					}
				}
 
			k= k+1;
			   }
		}
		else{				// if use certain constraint
			unsigned t = 0;
			for( std::map<QString, double(*)( vector<double> )>::iterator ii = optProbList[index]->constraints.begin(); ii!= optProbList[index]->constraints.end(); ++ii)
				{
				if(t== _mf->constraint){
 
					double step_size = 0.5 ;
					for(double x1= optProbList[index]->min[_mf->dim_indices[0]] ; x1 < optProbList[index]->max[_mf->dim_indices[0]] ; x1=x1+ step_size){
						for(double x2= optProbList[index]->min[_mf->dim_indices[1]] ; x2 < optProbList[index]->max[_mf->dim_indices[1]] ; x2=x2+ step_size){
							vector<double> coords(3,0);
							coords = _mf->fixed_values;
							coords[_mf->dim_indices[0]] = x1;
							coords[_mf->dim_indices[1]] = x2;
							double result = (*ii).second(coords);	// compute the y value
							if(result==1){
								// create PlotMarkers to visualize the infeasible area
								QwtPlotMarker *mark = new QwtPlotMarker();
									QwtSymbol *symb = new QwtSymbol();
									symb->setBrush(QBrush(Qt::red, Qt::SolidPattern));
									symb->setStyle(QwtSymbol::Ellipse);
									symb->setSize(5,5);
									mark->setSymbol(*symb);
									mark->setXValue(x1);						// set the position of marker on the plot
									mark->setYValue(x2);
									//mark->setZ(101);
									mark->attach(this);
							}
						}
					}
				}
				t= t+1;
				}
		}
	}
	//+++++++++++++++++ end of plot constraints curves if the option "use constraints" is selected ++++++++++++++++++++++
    replot();
}