The S3 methods defined are unclear

[xinyigua]

The methods defined are unclear given the code and its comments which has:

method for class 'data.frame' : 
interplot.plot <- function(...)

method for class 'lm' and 'glm' : 
interplot.default <- function(...)

The generic function method is also confusing and seems error prone / not compatible with the different classes and methods systems in R as well as class inheritance.