data
(ANY) the variable data, which can be any data object for which 
is.rectangular is TRUE,
such as a data.frame, matrix, or atomic vector. 
positions
(positions) the x values for the variables. 
start.position
(positions) the starting x value. 
end.position
(positions) the ending x value. 
future.positions
(positions) future x values used for predictions. 
units
(character) units for the data. 
title
(character) title of the data set. 
documentation
(character) user-supplied documentation. 
attributes
(ANY) attributes slot for arbitrary use.