Better remove all what you did so far and restart with something like that:

Qt Code: 
  1. class SpectrogramData: public QwtRasterData
  2. {
  3. public:
  4.     SpectrogramData()
  5.     {
  6.         m_intervals[ Qt::XAxis ] = QwtInterval( 0, 120 );
  7.         m_intervals[ Qt::YAxis ] = QwtInterval( 0, 120 );
  8.         m_intervals[ Qt::ZAxis ] = ...
  9.     }
  10.  
  11.     virtual QwtInterval interval( Qt::Axis axis ) const
  12.     {
  13.         if ( axis >= 0 && axis <= 2 )
  14.             return m_intervals[ axis ];
  15.  
  16.         return QwtInterval();
  17.     }
  18.  
  19.     virtual double value( double x, double y ) const
  20.     {
  21.         double value = ...; // find the value corresponding to x/y in your vector
  22.         return value;
  23.     }
  24.  
  25. private:
  26.     QwtInterval m_intervals[3];
  27. };
To copy to clipboard, switch view to plain text mode 

If you don't insist on using std::vector you could use QwtMatrixRasterData like demonstrated in the rasterview example. You get at least bilinear interpolation and a valid pixel hint for free.

Uwe