ggfun: Apply ggplot2 and extend some parts of it

#' To Draw a Archimedean Spiral
#'
#'
#' @param mapping Set of aesthetic mappings created by [aes()] or
#'   [aes_()]. If specified and `inherit.aes = TRUE` (the
#'   default), it is combined with the default mapping at the top level of the
#'   plot. You must supply `mapping` if there is no plot mapping.
#' @param data The data to be displayed in this layer. There are three
#'    options:
#'
#'    If `NULL`, the default, the data is inherited from the plot
#'    data as specified in the call to [ggplot()].
#'
#'    A `data.frame`, or other object, will override the plot
#'    data. All objects will be fortified to produce a data frame. See
#'    [fortify()] for which variables will be created.
#'
#'    A `function` will be called with a single argument,
#'    the plot data. The return value must be a `data.frame`, and
#'    will be used as the layer data.
#' @param geom The geometric object to use display the data
#' @param position Position adjustment, either as a string, or the result of
#'    a call to a position adjustment function.
#' @param na.rm If `FALSE`, the default, missing values are removed with
#'    a warning. If `TRUE`, missing values are silently removed.
#' @param show.legend logical. Should this layer be included in the legends?
#'   `NA`, the default, includes if any aesthetics are mapped.
#'   `FALSE` never includes, and `TRUE` always includes.
#'   It can also be a named logical vector to finely select the aesthetics to
#'   display.
#' @param inherit.aes If `FALSE`, overrides the default aesthetics,
#'   rather than combining with them. This is most useful for helper functions
#'   that define both data and aesthetics and shouldn't inherit behaviour from
#'   the default plot specification, e.g. [borders()].
#' @param ... Other arguments passed on to [layer()]. These are
#'    often aesthetics, used to set an aesthetic to a fixed value, like
#'    `colour = "red"` or `size = 3`. They may also be parameters
#'    to the paired geom/stat.
#' @export
#' @importFrom  ggplot2 layer
#' @examples
#' library(ggplot2)
#' ggplot() +
#'   stat_ars(aes(a = 10, b = 20, n = 2), col = "blue")

stat_ars <- function(mapping = NULL, data = NULL, geom = "point",
                     position = "identity", na.rm = FALSE, show.legend = NA,
                     inherit.aes = TRUE, ...) {
  layer(
    stat = StatArs, data = data, mapping = mapping, geom = geom,
    position = position, show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
}

#' @format NULL
#' @usage NULL
#' @importFrom ggplot2 ggproto

StatArs <- ggproto("StatArs", Stat,

                   compute_group = function(data, scales) {

                     theta <- seq(0, 2* data$n * pi, length = data$n * 1000)
                     r <- data$a + data$b * theta
                     x <- r * sin(theta)
                     y <- r * cos(theta)

                     data.frame(x = x, y = y)
                   },
                   required_aes = c("a", "b", "n")
)