Assuming that FunctionData is taken from the sinusplot example the code above doesn't make much sense - together with the rest of your question.

Better take this code for learning the very basics of how to show a curve on a QwtPlot: http://qwt.svn.sourceforge.net/viewv...75&view=markup.

Uwe

Qt Code: 
  1. int main( int argc, char **argv )
  2. {
  3.  	QApplication a( argc, argv );
  4.  
  5. 	QwtPlot plot;
  6.  	plot.setTitle( "Plot Demo" );
  7.  	plot.setCanvasBackground( Qt::white );
  8.  	plot.setAxisScale( QwtPlot::yLeft, 0.0, 10.0 );
  9.  	plot.insertLegend( new QwtLegend() );
  10.  
  11.  	QwtPlotGrid *grid = new QwtPlotGrid();
  12.  	grid->attach( &plot );
  13.  
  14.  	QwtPlotCurve *curve = new QwtPlotCurve();
  15.  	curve->setTitle( "Some Points" );
  16.  	curve->setPen( QPen( Qt::blue, 4 ) ),
  17.  	curve->setRenderHint( QwtPlotItem::RenderAntialiased, true );
  18.  
  19.  	QwtSymbol *symbol = new QwtSymbol( QwtSymbol::Ellipse,
  20.  	QBrush( Qt::yellow ), QPen( Qt::red, 2 ), QSize( 8, 8 ) );
  21.  	curve->setSymbol( symbol );
  22.  
  23.  	QPolygonF points;
  24.  	points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
  25.  	    << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
  26.  	    << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
  27.  	curve->setSamples( points );
  28.  
  29.  	curve->attach( &plot );
  30.  
  31.  	plot.resize( 600, 400 );
  32.  	plot.show();
  33.  
  34.  	return a.exec();
  35.  }
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