#' @param legend May be the two objects described below or the default \code{NULL} value.  The legend position can be moved by using ggplot2's theme element \code{pm + theme(legend.position = "bottom")} \describe{\item{a numeric vector of length 2}{provides the location of the plot to use the legend for the plot matrix's legend. Such as \code{legend = c(3,5)} which will use the legend from the plot in the third row and fifth column}\item{a single numeric value}{provides the location of a plot according to the display order. Such as \code{legend = 3} in a plot matrix with 2 rows and 5 columns displayed by column will return the plot in position \code{c(1,2)}}\item{a object from \code{\link{grab_legend}()}}{a predetermined plot legend that will be displayed directly}}
