The bridge from the array metadata into the density machinery of the plots. Mizer arrays are indexed by the model's weight grid, so a stored density is always a density with respect to weight; the other measures in density_measures arise only for quantities that the spectrum plots compute on the fly, such as a density per logarithmic weight.
array_density_wrt(x)"w" if the array holds a density, otherwise NA_character_.
A mizer array object.