Hi, this is the complete code for the part releted to the plot, I added setOrientation and setStyle to check your suggestion:

Qt Code: 
  1. int samplesNumber=100;
  2.     double yValue[samplesNumber], xValue[samplesNumber];
  3.     double tempf;
  4.     for (int i = 0; i < samplesNumber; i++){
  5.         tempf = (double) (i-5)/5;
  6.         yValue[i] = sin(tempf);
  7.         xValue[i] = double(i);
  8.     }
  9.  
  10.     QwtPlot * grafico;
  11.     QwtPlotCurve * curve = new QwtPlotCurve("Curve");
  12.     grafico = new QwtPlot(this);
  13.     curve->setOrientation(Qt::Vertical);
  14.  
  15.     curve->setSamples(xValue, yValue, samplesNumber);
  16.     curve->setPen(QColor(Qt::blue));
  17.     curve->setStyle(QwtPlotCurve::Steps);
  18.  
  19.     curve->setBrush(QBrush(Qt::blue, Qt::SolidPattern));
  20.     curve->setBaseline(0);
  21.  
  22.     curve->attach(grafico);
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