dc_obj: A deltacomp_obj object returned from the function predict_delta_comps
comp_total: A numeric scalar that is the original units of the composition to make the x-axis the original scale instead of in the range [min(delta), max(delta)] in (-1, 1).
units_lab: Character string of the units of the compositions relating to comp_total to add to the x-axis label
Returns
Returns a plot object from the ggplot2 package (that is, class of gg and ggplot).