geom_rtile <- function(mapping = NULL, data = NULL,
                      stat = "identity", position = "identity",
                      radius = grid::unit(6, "pt"),
                      ...,
                      na.rm = FALSE,
                      show.legend = NA,
                      inherit.aes = TRUE) {
  ggplot2::layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = GeomRtile,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      radius = radius,
      na.rm = na.rm,
      ...
    )
  )
}

GeomRtile <- ggplot2::ggproto("GeomRtile", GeomRrect,

  extra_params = c("na.rm", "width", "height"),

  setup_data = function(data, params) {
    data$width <- data$width %||% params$width %||% ggplot2::resolution(data$x, FALSE)
    data$height <- data$height %||% params$height %||% ggplot2::resolution(data$y, FALSE)

    transform(data,
      xmin = x - width / 2,  xmax = x + width / 2,  width = NULL,
      ymin = y - height / 2, ymax = y + height / 2, height = NULL
    )
  },

  default_aes = ggplot2::aes(
    fill = "grey20", colour = NA, size = 0.1, linetype = 1, alpha = NA
  ),

  required_aes = c("x", "y"),

  draw_key = ggplot2::draw_key_polygon

)