QWT- Help with plotting data from a vector.

[rockinfresh]

Hi all,

I am a newbie in QWT. Just wish to ask how can I plot data from a vector. The code below shows what I have done so far. Basically, the data I am storing into the vector are pixel values of an image along the axis. After that, I proceed to plot them, but there is always an error. I am not sure how to exactly do them either:/ I been trying to plot for almost 2 weeks plus already. But to no avail. I am still confused over the curve as well, why is there another vector needed for the setSamples() of curve?

vector<byte> v_char; //values of pixels taken from y-axis //tried double and int as well.
 
	//store pixel values into v_char
	v_char.reserve(image.rows * image.cols);
 
	for (int i = 0; i < image.rows; i++)
	{
		  int segment_start = *(uchar*)image.data + i * image.step;
		  v_char.insert(v_char.end(), segment_start, segment_start + image.cols);
	}
 
	//plotting of v_char vector
	QwtPlot *plota=new QwtPlot(); //Create plot widget
    plota->setTitle( "Plot Analysis" ); //Name the plot
    plota->setCanvasBackground( Qt::black ); //Set the Background colour
    plota->setAxisScale( QwtPlot::yLeft, 0, 1000000 ); //Scale the y-axis
    plota->setAxisScale(QwtPlot::xBottom,0,image.rows); //Scale the x-axis
    plota->insertLegend(new QwtLegend()); //Insert a legend
 
	//attach curve to plota
	QwtPlotCurve *curvea = new QwtPlotCurve(); // Create a curve
   // curvea->setTitle("Count"); //Name the curve
    curvea->setPen( Qt::white, 2);//Set colour and thickness for drawing the curve 
    //Use Antialiasing to improve plot render quality
    curvea->setRenderHint( QwtPlotItem::RenderAntialiased, true );
 
	QPolygonF points;
    for( int h = 0; h < v_char.size(); ++h) 
	{
        float bin_value = v_char[h];
        points << QPointF((float)h, bin_value);
    }
    curvea->setSamples(points); //pass points to be drawn on the curve
    curvea->attach( plota ); // Attach curve to the plot 
 
	plota->resize( 600, 400 ); //Resize the plot
	//plot.replot();
    plota->show(); //Show plot

vector<byte> v_char; //values of pixels taken from y-axis //tried double and int as well.
	
	//store pixel values into v_char
	v_char.reserve(image.rows * image.cols);

	for (int i = 0; i < image.rows; i++)
	{
		  int segment_start = *(uchar*)image.data + i * image.step;
		  v_char.insert(v_char.end(), segment_start, segment_start + image.cols);
	}

	//plotting of v_char vector
	QwtPlot *plota=new QwtPlot(); //Create plot widget
    plota->setTitle( "Plot Analysis" ); //Name the plot
    plota->setCanvasBackground( Qt::black ); //Set the Background colour
    plota->setAxisScale( QwtPlot::yLeft, 0, 1000000 ); //Scale the y-axis
    plota->setAxisScale(QwtPlot::xBottom,0,image.rows); //Scale the x-axis
    plota->insertLegend(new QwtLegend()); //Insert a legend

	//attach curve to plota
	QwtPlotCurve *curvea = new QwtPlotCurve(); // Create a curve
   // curvea->setTitle("Count"); //Name the curve
    curvea->setPen( Qt::white, 2);//Set colour and thickness for drawing the curve 
    //Use Antialiasing to improve plot render quality
    curvea->setRenderHint( QwtPlotItem::RenderAntialiased, true );
	
	QPolygonF points;
    for( int h = 0; h < v_char.size(); ++h) 
	{
        float bin_value = v_char[h];
        points << QPointF((float)h, bin_value);
    }
    curvea->setSamples(points); //pass points to be drawn on the curve
    curvea->attach( plota ); // Attach curve to the plot 

	plota->resize( 600, 400 ); //Resize the plot
	//plot.replot();
    plota->show(); //Show plot

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If anybody do know, please please do help, I already at my wits end. I can't seem to find an example or tutorial online for QWT plots. I already tried lots of things... Is there a problem with my code? Or my whole programming flow is wrong? Please help. Thanks.