The IPA vowel chart is already 3D, since there's rounding.
I suspect a complex vowel would involve a waveform that's based on exponential curves rather than sine waves. If it's not purely imaginary, it might grow exponentially in volume.
The comic itself seems to be partly imaginary, since the direct link is giving me a 404; only the front page works. [Edit: Looks like this has been fixed now.]