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#' Find the Coordinate from the Scale of a Nomograph
#'
#' Find the x and y coordinate corresponding to a specified point on a
#' nomograph scale.
#' @param df
#' A data frame with three columns, the \code{x} and \code{y} coordinates
#' of the nomograph scale and the corresponding values.
#' @param val
#' A numeric scalar identifying the point on the scale for which the
#' coordinates will be returned.
#' @export
#' @import
#' plotrix
#' @details
#' The function makes it easier to add points or lines to a nomograph for
#' illustrative purposes.
#'
#' Each scale is assumed to be displayed on the log10 scale.
#' @return
#' A numeric vector of length two with the x and y coordinates of the
#' specified point in plotting units of the nomograph.
#' @examples
#' scales <- LWnomo1(TRUE)
#' fromxy <- nomoCoord(scales$scale1r, 34)
#' toxy <- nomoCoord(scales$scale3, 16^2/(100*34))
#' segments(fromxy[1], fromxy[2], toxy[1], toxy[2], col="red")
#'
nomoCoord <- function(df, val) {
if(val < df$values[1] | val > df$values[2]) {
stop("val must be between ", df$values[1], " and ", df$values[1], ".")
}
c(df$x[1], rescale(log10(c(val, df$values)), df$y)[1])
}
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