Hello, I'm trying to learn how to use Qwt and I'm stucked with one problem.

When I create a custom application with just main function everything works fine. (the code is based on examples from qwt)

Qt Code: 
  1. int main(int argc, char *argv[])
  2. {
  3.     QApplication a(argc, argv);
  4.  
  6.     double x[100];
  7.     double y[100];
  8.  
  9.     for(int i=0;i<100;i++)
  10.     {
  11.         x[i] = i/20.0;
  12.         y[i] = sin(x[i]);
  13.     }
  14.     QHBoxLayout *layout = new QHBoxLayout(&w);
  15.     layout->setContentsMargins( 0, 0, 0, 0 );
  16.     QwtPlot *myPlot = new QwtPlot(&w);
  17.     myPlot->setMinimumSize(w.geometry().width()-20,w.geometry().height()-20);
  18.     // add curve
  19.     QwtPlotCurve *curve = new QwtPlotCurve();
  20.     curve->setRawSamples(x, y, 100);
  21.     curve->setSymbol(new QwtSymbol(QwtSymbol::Cross, Qt::NoBrush,
  22.         QPen(Qt::black), QSize(5, 5) ) );
  23.     curve->setPen(QColor(Qt::darkGreen));
  24.     curve->setStyle(QwtPlotCurve::Lines);
  25.     curve->setCurveAttribute(QwtPlotCurve::Fitted);
  26.     curve->attach(myPlot);
  27.     QPushButton *b = new QPushButton(&w);
  28.     layout->addWidget(myPlot);
  29.     layout->addWidget(b);
  30.     w.setLayout(layout);
  31.     w.show();
  32.  
  33.     return a.exec();
  34. }
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The problem is when i try to copy this part of code and put it in MainWindow designed in QtCreator.

Qt Code: 
  1. void MainWindow::initGraph()
  2. {
  3.     double x[100];
  4.     double y[100];
  5.  
  6.     for(int i=0;i<100;i++)
  7.     {
  8.         x[i] = i/100.0;
  9.         y[i] = sin(x[i]);
  10.     }
  11.     QHBoxLayout *layout = new QHBoxLayout(ui->widget);
  12.     layout->setContentsMargins( 0, 0, 0, 0 );
  13.     QwtPlot *myPlot = new QwtPlot(ui->widget);
  14.     myPlot->setMinimumSize(ui->widget->geometry().width()-20,ui->widget->geometry().height()-20);
  15.     myPlot->updateGeometry();
  16.     // add curves
  17.     QwtPlotCurve *curve = new QwtPlotCurve();
  18.     curve->setRawSamples(x, y, 100);
  19.     curve->setSymbol(new QwtSymbol(QwtSymbol::Cross, Qt::NoBrush,
  20.         QPen(Qt::black), QSize(5, 5) ) );
  21.     curve->setPen(QColor(Qt::darkGreen));
  22.     curve->setStyle(QwtPlotCurve::Lines);
  23.     curve->setCurveAttribute(QwtPlotCurve::Fitted);
  24.     curve->attach(myPlot);
  25.     myPlot->autoReplot();
  26.     myPlot->replot();
  27.     myPlot->autoRefresh();
  28.     layout->addWidget(myPlot);
  29.     ui->widget->setLayout(layout);
  30. }
  31.  
  32. //and main 
  33.  
  34. int main(int argc, char *argv[])
  35. {
  36.     QApplication a(argc, argv);
  37.     MainWindow w;
  38.     w.initGraph();
  39.     w.show();
  40.  
  41.     return a.exec();
  42. }
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After running code above I see my app, axis of the plot are scaled but curve is not painted properly (I think only first point is painted). I tried couple of functions from plot to fix it (as you can see in code above). I've created 2 or 3 gui apps using qtCreator's designer and even with just 1 main widget it doesn't work fine. Could anyone tell me where do I make a mistake and how to fix that problem?

Sorry if someone finds it hard to understand what I've written here but english is not my native language.