//buf.h
#ifndef BUF_H
#define BUF_H

#include <qwt_plot.h>

class buf : public QwtPlot
{
    Q_OBJECT

public:
    buf(QWidget *parent);
    ~buf();

private:
    
};

#endif // BUF_H

//buf.cpp
#include "buf.h"

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

}

buf::~buf()
{

}

class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict
{
    Q_OBJECT
.......   
    explicit QwtPlot(QWidget * = NULL);
    explicit QwtPlot(const QwtText &title, QWidget *p = NULL);
..........
};

moc_SpectrViewer.obj : error LNK2001: неразрешенный внешний символ ""public: static struct QMetaObject const QwtPlot::staticMetaObject" (?staticMetaObject@QwtPlot@@2UQMetaObject@@"
F:\CurrentProjects\RTCalculator\Debug\RTCalculator.exe : fatal error LNK1120: 1 неразрешенных внешних элементов

class CSpectrViewer : public QwtPlot
    {
        Q_OBJECT

.....