plots for the interaction model
# S3 method for im_mst
plot(x, item_id = NULL, show.observed = TRUE, curtains = 10, zoom = FALSE, ...)output of fit_inter_mst
id of the item to plot
plot the observed mean item scores for each test score
percentage of most extreme values to cover with curtains, 0 to omit curtains
if TRUE, limits the plot area to the test score range allowed by the routing rules
further arguments to plot