… make sure that the metaphor serves the data and not vice versa.
The writer is warning against finding a metaphor that, although it doesn't quite fit the data being communicated, is so vivid and compelling that you are tempted to distort the data so that the metaphor can be employed.
… one must avoid concretizing metaphors.
Don't lose track of the fact that a metaphor is an abstraction, a symbolic illustration, not to be confused with the way the aspect of the real world under discussion actually works.
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