an object of class "pair" as a
result from function pair.
itemnumber
an integer, defining the number of the
item to plot the respective categoy probability for. This
is set to an arbitrary default value of itemnumber
= 1 to avoid error messages when you forget to choose an
item to plot the expected sco
ra
an integer, defining the (logit) range for
x-axis
plot
a logical (default plot = TRUE),
defining wether to supress plotting an just return a
matrix of category probabilities