qwt plots & inheritance

[fatecasino]

what is the difference between:

my2dPlot::my2dPlot(QWidget *parent)
    : QwtPlot(parent)
{

my2dPlot::my2dPlot(QWidget *parent)
    : QwtPlot(parent)
{

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and

my2dPlot::my2dPlot(QWidget *parent)
    : QWidget(parent)
{

my2dPlot::my2dPlot(QWidget *parent)
    : QWidget(parent)
{

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the first appears as the screenshot of the first post
and the second appears as the screenshot f this post. It seems strange but there's something definitely wrong. The bode mouse tracker works only in grey area just above the green plot. The green plot is the one that is constructed in the my2dplot class. I can load a data file and see the plot, BUT I cannot use the zoomer of the bode example! Although I have declared:

class MyWidget : public QWidget
{
public:
    MyWidget(QWidget *parent = 0);
    my2dPlot *mainPlot;

class MyWidget : public QWidget
{
public:
    MyWidget(QWidget *parent = 0);
    my2dPlot *mainPlot;

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which means that mainPlot is a my2dPlot type and not just QwtPlot type. Somehow 2 different plot objects(my2dPlot,QwtPlot) are constructed at the same exactly position(see attachment)!
Any ideas?!