The axis x is a psychic distance. The axis y is activation of the organism. Abrupt changes of behavior and rhetorics take place in this 2D space. The curves connects points, where this change of behavior take place. For instance, the curve of attraction says, where the desire for the other person is switched on and off. When he touches this curve, he would experience sudden afflux of love. When he does not touch the curve for the long time, the person can come to a erroneous conclusion, the he does not love the other.
More information you can find in the book Partners and breakup.
A concise overview of preferential curves is on: http://preferentialcurves.klimes.us