Internal function to compute 2D marginal compositions for plots
gsi.plotmargin(X,d,margin,what="data")a multivariate compositional dataset
a numeric or character vector of two elements specifying the parts to be kept in the marginalization process
a character specifying the type of margin to be chosen. Possible values are "acomp", "rcomp" or a column name from the dataset.
simply here to avoid an error if the argument is passed.
a composition of three elements.