The only way to change the “Plot Spectrum” in the Audacity “Analyse” menu is adding new functions to the Audacity C/C++ source code.
The builtin Audacity Nyquist FFT functions are broken (see this thread), so writing a plugin with already builtin Audacity Nyquist functions is currently not possible and I have no idea when this will be fixed.
The only workaround I know is using the Spectutils package together with GNU Octave (free MatLab clone) and Gnuplot to make your own 2D and 3D FFT spectrograms.
- edgar