NEP 42 mentions physical units as a possible use case for the new DType. Having worked on `unyt`, which is an ndarray subclass, and other unit system implementations based on the dispatch mechanism, I am quite familiar with the challenges. One challenge is integration with Matplotlib that has a unit conversion interface. This interface is invoked by a registry mapping from unit array type, such as `unyt_array`, and its conversion class. A custom DType will still result in an ndarray type - correct? If so, Matplotlib will attempt to perform its internal calculations eventually leading to an invalid operation when adding a physical quantity to a pure number. This happens today with `unyt_array` and some of the plotting functions that allow the subclass to leak through into the internals. Has this been discussed? I can imagine other libraries will have the same challenge.
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