pathdiagram: Basic functions for drawing path diagrams

#' @title Open a new frame for a path diagram
#' 
#' @description
#' Use this function to open a white canvas to start drawing a path diagram. 
#' By default, \code{wall} opens a new plot window from 0 to 1 in both axes.
#' 
#' @details
#' \code{wall} calls \code{\link{plot.new}} and \code{\link{plot.window}}
#' to open a new plot frame.
#'
#' @param xlim Numeric vector of length 2 giving the x coordinate range.
#' Default \code{xlim = c(0, 1)}.
#' @param ylim Numeric vector of length 2 giving the y coordinate range.
#' Default \code{ylim = c(0, 1)}.
#' @param xpd Logical value to indicate if all plotting is clipped to the 
#' figure region. 
#' @param ... other graphical arguments passed on to \code{\link{plot.window}}.
#' @author Gaston Sanchez
#' @seealso \code{\link{manifest}}, \code{\link{latent}}, \code{\link{draw}}
#' @export
#' @examples
#'
#'  \dontrun{
#'  # latent variables
#'  attack = latent("ATTACK", x=0.35, y=0.7, rx=0.08, ry=0.06)
#'  defense = latent("DEFENSE", x=0.35, y=0.3, rx=0.08, ry=0.06)
#'  success = latent("SUCCESS", x=0.65, y=0.5, rx=0.08, ry=0.06)
#'
#'  # open diagram
#'  wall()
#'  
#'  # draw latent variables
#'  draw(attack)
#'  draw(defense)
#'  draw(success)
#'  
#'  # add arrows
#'  arrow(from=attack, to=success, start="east", end="west")
#'  arrow(from=defense, to=success, start="east", end="west")
#'  }
#'
wall <-
function(xlim = c(0, 1), ylim = c(0, 1), xpd = TRUE, ...)
{
  plot.new()
  plot.window(xlim = xlim, ylim = ylim, xpd = xpd, ...)
}