#' @export
geom_twoway_bar <- function(mapping = NULL, data = NULL,
                     stat = "identity",
                     width = NULL,
                     ...,
                     na.rm = FALSE,
                     show.legend = NA,
                     inherit.aes = TRUE) {

  layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = GeomTwowayBar,
    position = "stack",
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      width = width,
      na.rm = na.rm,
      ...
    )
  )
}

#' @rdname ggalt-ggproto
#' @format NULL
#' @usage NULL
#' @export
GeomTwowayBar <- ggproto("GeomTwowayBar", GeomRect,
  required_aes = c("x", "y"),

  do_setup_data = function(data, params) {
    data$width <- data$width %||%
      params$width %||% (resolution(data$x, FALSE) * 0.9)

    d_plus <- subset(data, y>=0, drop=FALSE)
    d_minus <- subset(data, y<0, drop=FALSE)

    d_plus <- transform(d_plus,
      ymin = pmin(y, 0), ymax = pmax(y, 0),
      xmin = x - width / 2, xmax = x + width / 2, width = NULL,
      is_plus = TRUE
    )

    d_minus <- transform(d_minus,
      ymin = pmin(y, 0), ymax = pmax(y, 0),
      xmin = x - width / 2, xmax = x + width / 2, width = NULL,
      y = abs(y),
      is_plus = FALSE
    )

    cat("setup_data() after _________\n")
    print(rbind(d_plus, d_minus))

    rbind(d_plus, d_minus)

  },

  draw_panel = function(self, data, panel_scales, coord, width=NULL) {

    cat("draw_panel() _________\n")
    print(data)
#
#     d_plus <- subset(data, is_plus)
#     d_minus <- subset(data, !is_plus)
#     d_minus$y <- -d_minus$y

    gList(
      ggplot2::ggproto_parent(GeomBar, self)$draw_panel(data, panel_scales, coord)
    )
  }
)