is it a C++ error or Qt/Qwt related?

[mcostalba]

After

redCurve.setRawData(xval,redCurveData,size);
greenCurve.setRawData(xval,greenCurveData,size);

redCurve.setRawData(xval,redCurveData,size);
greenCurve.setRawData(xval,greenCurveData,size);

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you don't need the previous samples anymore (it seems) so you can call freeData() after that, of course delaying assign of new pointers until after the free of old data:

void Dialog::getData()
{
    int size = 1+rand()%10 + 10;
 
    double* redCurveDataPtr = new double[size];
    double* greenCurveData = new double[size];
    double* xval = new double[size];
 
    for(int i = 0; i < size; i++)
    {
        redCurveData[i] = 0+rand()%10;
        greenCurveData[i] = 0+rand()%10;
        xval[i] = i + 1;
    }
 
    redCurve.setRawData(xval,redCurveData,size);
    greenCurve.setRawData(xval,greenCurveData,size);
 
    // old data is no more needed now
    freeData();
 
    // now we can add newly created samples pointers
    samplesPtr << redCurveDataPtr << greenCurveData << xval;
 
    plot->setAxisScale(QwtPlot::xBottom, 1, size);
    plot->setAxisScale(QwtPlot::yLeft, 0, 10);
 
    drawCurves();
}

void Dialog::getData()
{
    int size = 1+rand()%10 + 10;

    double* redCurveDataPtr = new double[size];
    double* greenCurveData = new double[size];
    double* xval = new double[size];

    for(int i = 0; i < size; i++)
    {
        redCurveData[i] = 0+rand()%10;
        greenCurveData[i] = 0+rand()%10;
        xval[i] = i + 1;
    }

    redCurve.setRawData(xval,redCurveData,size);
    greenCurve.setRawData(xval,greenCurveData,size);

    // old data is no more needed now
    freeData();

    // now we can add newly created samples pointers
    samplesPtr << redCurveDataPtr << greenCurveData << xval;

    plot->setAxisScale(QwtPlot::xBottom, 1, size);
    plot->setAxisScale(QwtPlot::yLeft, 0, 10);

    drawCurves();
}

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[mcostalba]

A couple of bugs

Because pointer vector is reused freeData() should be:

void Dialog::freeData()
{
   for(int i = 0; i < samplesPtr.size(); i++)
        delete[] samplesPtr.at(i);
 
   samplesPtr.clear(); 
}

void Dialog::freeData()
{
   for(int i = 0; i < samplesPtr.size(); i++)
        delete[] samplesPtr.at(i);

   samplesPtr.clear(); 
}

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And also remember to add a freeData() call in the d'tor to be sure the last samples data set is properly released when closing the dialog.

[jiveaxe]

Inserting freeData() where you say gives me strange graphs. Removing it works again right.

Regards