As for the feature request of letting you visualize all channels as FFTs of each channel, I meant this: (BTW, I've updated this interactive mockup to add this, which already exists on the similar spectrum analyzer plugin for another music player except for the M/S part)
[attach type=image]26984[/attach]
And for Mid/Side part (which can be easily done using M/S encoding then doing FFTs of resulting M/S signal for visualization):
[attach type=image]26986[/attach]

Since the real and imaginary parts of the transform are orthogonal, this is the same as performing a real-valued FFT once for each input and then combining the individual outputs. 

See this derivation for a more detailed explanation:  http://www.hyperdynelabs.com/dspdude/papers/COMPUTING%20THE%20FFT%20OF%20TWO%20REAL%20SIGNALS%20USING%20A%20SINGLE%20FFT.pdf

const j = complexSpectrum.length-i;
      x = idxWrapOver(i, complexSpectrum.length),
      y = idxWrapOver(j, complexSpectrum.length);
spectrum1[i] = Math.hypot(complexSpectrum[x].re+complexSpectrum[y].re, complexSpectrum[x].im-complexSpectrum[y].im)/Math.SQRT2;
spectrum2[i] = Math.hypot(complexSpectrum[x].im+complexSpectrum[y].im, complexSpectrum[x].re-complexSpectrum[y].re)/Math.SQRT2;

function idxWrapOver(x, length) {
  return (x % length + length) % length;
}

Which codecs are these?
...