In fact, most diagrams just aren't that rigorous. They employ rules or generalizations with qualifications and exceptions, and they are judged by their functionality, not the mechanistic precision of the diagram itself. Even the rigorous consistency of formal logic has to be taken with a grain of salt when it comes to actually using it. Any general assertion we make about experience, scientific laws included, is going to have qualifications and exceptions.
More important, though, applying any kind of diagram, rigorous or less so, is not a mechanical process either. Take for instance a moral question like pulling the plug:
Diagrammatic thinking mediates thought with consistency, but it is not limited to a mechanistic approach because of that.