I find the XY screen to be interesting, but not necessarily useful.
The Deus XY screen is mapping a series of impedance vectors to draw what is called a 'phase envelope'. The more a target's impedance vector changes phase over a given sweep, the 'wider' the phase envelope, and thus the greater harmonic content in the tone. So the harmonic content produced by the machine is inversely proportional to the 'eccentricity' of the ellipse (or directly proportional to the length of the semi-minor axis). Larger objects tend to produce more phase angle variation over a given sweep, and therefore produce a less eccentric phase envelope and thus produce more harmonic content in their tone. So the amount of harmonic content in a target's response can be indicative of both it's size and uniformity.
(for those wondering what I am referring to when I say 'harmonic content', try this: first whistle, that is a clean tone with no harmonic content. Now try to whistle, but instead of just blowing out air, hum at the same time. If you do it right, you will then be creating a whistle with harmonic content. It sounds almost like a spaceship sound [since the frequency of the whistle and the frequency of the hum are different, yet combined into one tone])
All of this information is arguably redundant and can be heard through the tones. I haven't been able to extract any information from the XY screen, that isn't encoded in the audio, that improves my decision making on to dig or not dig.
