#'
#' Add a Legend to a Plot
#'
#' *Internal function*. Add a legend to a plot created by either \code{ChemoSpec} or \code{ChemoSpec2D}.
#' \code{ChemoSpec2D} does not use symbols.
#'
#' @param spectra `r .writeDoc_Spectra3()`
#'
#' @param use.sym Logical; if true, the color scheme is set to black and the
#' points plotted with symbols. Applies only to \code{ChemoSpec}.
#'
#' @param leg.loc `r .writeDoc_LegLoc()`
#'
#' @param \dots `r .writeDoc_GraphicsDots()`
#'
#' @return None. Side effect is to modify an existing plot.
#'
#' @author `r .writeDoc_Authors("BH")`
#' @importFrom graphics legend
#' @export
#' @keywords internal
#'
.addLegend <- function(spectra, leg.loc, use.sym, ...) {
.chkArgs(mode = 0L)
if (inherits(spectra, "Spectra")) gr <- sumGroups(spectra)
if (inherits(spectra, "Spectra2D")) {
if (use.sym) stop("use.sym cannot be used with ChemoSpec2D")
gr <- sumGroups(spectra)
}
leg.txt <- c("Key", gr$group)
leg.col <- c("black", gr$color)
if (use.sym) leg.col <- "black"
leg.pch <- NA
if (use.sym) leg.pch <- c(NA, gr$sym)
legend(leg.loc, leg.txt, text.col = leg.col, cex = 0.75, pch = leg.pch, ...)
} # end of addLegend
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