FFTPHS(freqexpr)

Returns the Fast Fourier Transform phase value corresponding to
'freqexpr' (-NYQUIST <= freqexpr <= NYQUIST) determined by the most
recent invocation of the FFT procedure.  FFTPHS is calculated as
ARCTAN(FFTIM(freqexpr)/FFTRE(freqexpr)).  Using FFTPHS in an expression
does not automatically set the Y axis scaling values.
