I have had time to further look at my problem and find the solution.
In reality I didn´t see that the line of the curve within the legend was plotted, because the line has a light yellow colour and the background is light gray. The line is also very short.
I added a curve title which is now displayed as assumed and so fix the view onto the legend.
I have attached the revised code below.

Uwe, I am aware that in a normal case it is necessary to allocate the memory on the heap, this was a fast evaluation how to mix SigLib and Qwt, and I only had a main function, so allocating on the stack was OK for that peculiar purpose.
Anyway, now I am going to clone the SigLib graphics function written with SignalVisualize/wxWidgets with Qt/Qwt, and there of course I´ll use the approach recommended by you.
Qt Code: 
  1. // plot object
  2. 	QwtPlot myPlot;
  3. 	myPlot.setTitle( "Autospektrum!" );
  4. 	myPlot.setAxisTitle( QwtPlot::xBottom, "frequency bin" );
  5. 	myPlot.setAxisTitle( QwtPlot::yLeft, "log Amplitude" );
  6. 	myPlot.setCanvasBackground( QColor( Qt::darkBlue ));
  7.  
  8. 	// grid 
  10.         grid.enableXMin( true );
  11.         grid.setMajPen( QPen( Qt::white, 0, Qt::DotLine ));
  12.         grid.setMinPen( QPen( Qt::gray, 0 , Qt::DotLine ));
  13.         grid.attach( &myPlot );
  14.  
  15. 	// define the curve object
  16. 	QwtPlotCurve curve;
  17. 	curve.setSamples( pXData, pResults, FFT_LENGTH );
  18. 	curve.setPen( QPen( Qt::yellow ));
  19. 	curve.setRenderHint( QwtPlotItem::RenderAntialiased );
  20. 	curve.setTitle("Sinus");
  21. 	curve.setLegendAttribute( QwtPlotCurve::LegendShowLine );
  22. 	curve.attach( &myPlot );
  23.  
  24. 	// legend
  25. 	QwtLegend legend;
  26. 	myPlot.insertLegend( &legend, QwtPlot::RightLegend );
  27.  
  28. 	//myPlot.replot();
  29. 	myPlot.show();
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