you have the solution in your code already:

Qt Code: 
  1. QwtPlot *plot=new QwtPlot();
  2.     plot->setTitle( "Plot Demo" );
  3.     plot->setCanvasBackground( Qt::white );
  4.     //plot->setAxisScale( QwtPlot::yLeft, 0.0, 20.0);
  5.    // plot->setAxisScale(QwtPlot::xBottom, 0.0, 20.0);
  6.     plot->insertLegend( new QwtLegend() );
  7.     plot->setAxisTitle(QwtPlot::xBottom, "X Axis");
  8.     plot->setAxisTitle(QwtPlot::yLeft, "Y Axis");
  9.     QwtPlotGrid *grid = new QwtPlotGrid();
  10.     grid->attach( plot );
  11.     grid->setPen( Qt::gray, 0.0, Qt::DotLine );
  12.  
  13.     QwtPlotCurve *curve = new QwtPlotCurve();
  14.     curve->setTitle( "Pixel Count" );
  15.     curve->setPen(QPen(Qt::blue, 3,Qt::SolidLine) );
  16.     curve->setRenderHint( QwtPlotItem::RenderAntialiased, true);
  17.     QwtSymbol *symbol = new QwtSymbol( QwtSymbol::Ellipse,QBrush(Qt::yellow),QPen(Qt::red,2),QSize(8,8));
  18.     curve->setSymbol(symbol );
  19.  
  20.     QPolygonF points;
  21.     curve->setSamples( points );
  22.     curve->attach( plot ); 
  23.  
  25.     QList<QByteArray> read_data;
  26.     b1= process->readAllStandardOutput();
  27.     read_data=b1.split('\n');
  28.     int i=0, n=read_data.count();
  29.     float y[100000];
  30.  
  31.     for( i=0; i<n; i++ )
  32.     {
  33.         QString result=read_data.at(i);
  34.         result.remove(0,10);
  35.         qDebug("%s",result.toLatin1().data());
  36.         y[i]=result.toLatin1().toFloat();
  37.         points << QPointF( i, y[i] );
  38.         // check wether curve is updated, if not do it here
  39.         plot->replot;
  40.         plot->show;
  41.     }
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Qwt is not installed on the computer I am writing from, so I cannot check my code, but the solution might not be far from here.

Just a remark. Converting a byte array to a string and then to a float is heavy, there should be a better way to do that. How fast is your data input rate?