A size spectrum is a density, and a density only has a meaning together with the variable it is a density with respect to. That variable is one of
"w"a density with respect to weight, e.g. numbers per gram.
"log_w"a density with respect to logarithmic weight, e.g. numbers per log weight interval.
"l"a density with respect to length, e.g. numbers per cm.
"log_l"a density with respect to logarithmic length.
NAnot a density, e.g. a rate or a dimensionless quantity. Such values are left alone when the size axis changes.
density_measuresA character vector of the four density measures.
Mizer arrays are indexed by the model's weight grid, so an array that holds
a density (type = "density", see array_types) always holds one with
respect to weight. The other measures arise for quantities the spectrum plots
compute on the fly: plotSpectra(per_log_size = TRUE) shows a density with
respect to logarithmic weight, and either can be restated per unit length by
size_axis = "l".