The specifics of different implementations of these visualisations vary a lot. This answer may therefore not accurately describe your tool, but should give you an idea of what's happening nonetheless.
These visualisations plot frequency against phase. An audio signal can be represented by the sum of several sine waves of different frequencies, amplitudes and phases. The phase analysis seperates a small section in time of the audio into these sine waves and compares the phase of each frequency component with that of the other channel. If they are in sync, a datapoint is added in the middle. If the left channel lags behind, a mark is drawn on the negative side and vice versa. If both channels oppose completely, i.e. the phase shift is 180°, the number is either -1 or 1. Those are of course equivalent, a sine shifted half a period to the right cannot be distinguished from one shifted left.
One of the applications of these analyses is merging channels, for instance when mixing to mono. Frequencies that are plotted in the center will increase in amplitude, whereas points far to the side of the plot will mostly cancel. If the distribution varies strongly between important frequencies, the resulting mix can sound very distorted. A narrow graph suggest both channels are very similar and there was very little stereo information to begin with.
A great way to experiment with these tools is to invert one of the channels and listen for changes in the mix and the visualisations.