object. 
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, which must be 
of type positionsNumeric. 
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) the units for the data. 
title
(character) the title of the data set. 
documentation
(character) user-supplied documentation. 
attributes
(ANY) the attributes slot for arbitrary use. 
units.position
(character) the units for the positions slot.