Hi, I'm trying some Qwt programming. I have made a simple program to show my problem: in the following application there are two checkboxes controlling if the respective curves are displayed and a pushbutton to get new data for the curves (here I have simulated the data with rand(), but in my real application they are retrieved from a database). The checkboxes are connected to drawCurves() which attach/detach curves to plot. Now if I click on the button the curves are displayed properly but if i toggle the checkboxes to show/hide a curve all curves gone.

Qt Code: 
  1. #include <QtGui>
  2. #include <qwt_plot.h>
  3. #include <qwt_plot_curve.h>
  4. #include <cstdlib>
  5. #include <ctime>
  6.  
  7. class Dialog: public QDialog
  8. {
  9. 	Q_OBJECT
  10.  
  11. 	public:
  12. 		Dialog(QWidget *parent = 0);
  13.  
  14. 	public slots:
  15. 		void getData();
  16. 		void drawCurves();
  17.  
  18. 	private:
  19. 		QwtPlot *plot;
  20. 		QwtPlotCurve redCurve;
  21. 		QwtPlotCurve greenCurve;
  22. 		QCheckBox *redCurveCheckBox;
  23. 		QCheckBox *greenCurveCheckBox;
  24. };
  25.  
  26. Dialog::Dialog(QWidget *parent)
  27. 	: QDialog(parent)
  28. {
  29. 	srand(time(0));
  30.  
  31. 	redCurveCheckBox = new QCheckBox("Red Curve");
  32. 	redCurveCheckBox->setChecked(true);
  33. 	greenCurveCheckBox = new QCheckBox("Green Curve");
  34. 	QPushButton *drawButton = new QPushButton("Draw");
  35.  
  36. 	plot = new QwtPlot;
  37.  
  38. 	redCurve.setPen(QPen(QColor(Qt::red),2));
  39. 	redCurve.setStyle(QwtPlotCurve::Lines);
  40. 	redCurve.setRenderHint(QwtPlotItem::RenderAntialiased);
  41.  
  42. 	greenCurve.setPen(QPen(QColor(Qt::green),2));
  43. 	greenCurve.setStyle(QwtPlotCurve::Lines);
  44. 	greenCurve.setRenderHint(QwtPlotItem::RenderAntialiased);
  45.  
  46. 	getData();
  47.  
  48. 	QVBoxLayout *rightLayout = new QVBoxLayout;
  49. 	rightLayout->addWidget(redCurveCheckBox);
  50. 	rightLayout->addWidget(greenCurveCheckBox);
  51. 	rightLayout->addWidget(drawButton);
  52. 	rightLayout->addStretch();
  53.  
  54. 	QHBoxLayout *mainLayout = new QHBoxLayout;
  55. 	mainLayout->addWidget(plot);
  56. 	mainLayout->addLayout(rightLayout);
  57.  
  58. 	setLayout(mainLayout);
  59. 	resize(640,480);
  60.  
  61. 	connect(redCurveCheckBox, SIGNAL(clicked()), this, SLOT(drawCurves()));
  62. 	connect(greenCurveCheckBox, SIGNAL(clicked()), this, SLOT(drawCurves()));
  63. 	connect(drawButton, SIGNAL(clicked()), this, SLOT(getData()));
  64. }
  65.  
  66. void Dialog::getData()
  67. {
  68. 	int size = 1+rand()%10 + 10;
  69.  
  70. 	double redCurveData[size];
  71. 	double greenCurveData[size];
  72. 	double xval[size];
  73.  
  74. 	for(int i = 0; i < size; i++)
  75. 	{
  76. 		redCurveData[i] = 0+rand()%10;
  77. 		greenCurveData[i] = 0+rand()%10;
  78. 		xval[i] = i + 1;
  79. 	}
  80.  
  81. 	redCurve.setRawData(xval,redCurveData,size);
  82. 	greenCurve.setRawData(xval,greenCurveData,size);
  83.  
  84. 	plot->setAxisScale(QwtPlot::xBottom, 1, size);
  85. 	plot->setAxisScale(QwtPlot::yLeft, 0, 10);
  86.  
  87. 	drawCurves();
  88. }
  89.  
  90. void Dialog::drawCurves()
  91. {
  92. 	if(redCurveCheckBox->isChecked())
  93. 		redCurve.attach(plot);
  94. 	else
  95. 		redCurve.detach();
  96.  
  97. 	if(greenCurveCheckBox->isChecked())
  98. 		greenCurve.attach(plot);
  99. 	else
  100. 		greenCurve.detach();
  101.  
  102. 	plot->replot();
  103. }
  104.  
  105. int main(int argc, char *argv[])
  106. {
  107. 	QApplication app(argc, argv);
  108. 	Dialog *dialog = new Dialog;
  109. 	dialog->show();
  110. 	return app.exec();
  111. }
  112.  
  113. #include "main.moc"
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Seems that the data associated to each curve is lost when un/checking: if I output the curves data during drawCurves() with this piece of code:

Qt Code: 
  1. for(int i = 0; i < 9; i++)
  2. 	std::cout << redCurve.y(i) << " ";
  3. std::cout << std::endl;
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after pushing the button I have, for example, 7 5 3 0 7 2 6 6 1, while un/checking one of the checkbox i have, for example

[HTML]3.30667e-317 -8.91495e+303 3.90058e-314 2.36344e-310 4.94066e-324 4.94066e-324 4.94066e-324 4.94066e-324 0[/HTML]

Maybe is a c++ error but I don't know where.

Regards