Halliday & Matthiessen (1999: 294-5):
The whole metaphorical elaboration [of the semantic system] is made possible by a fractal pattern that runs through the whole system. We have suggested that the metaphorical elaboration is a token–value relation; but in order for it to be a token–value relation within the semantic system, it has to be natural in the sense that the token and the value domains have to be similar enough to allow for the token to stand for the value. The principle behind this similarity is the fractal pattern of projection/expansion …
That is, while grammatical metaphor constitutes a move from one “phenomenal domain” to another … this move is made possible because fractal types engender continuity across these domains: the metaphorical move from one phenomenal domain to another takes place within the one and the same transphenomenal domain.