Extending ggplot2

library(ggplot2)

StatMinus <- ggproto(
  'StatMinus',
  Stat,
  required_aes = c('x', 'y'),
  compute_group = function(data, scales) {
    ind <- data$y < 0
    data[ind, , drop = FALSE]
  }
)

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

a <- data.frame(x = 1:10, y = c(rep(-1, 5), rep(1, 5)))
ggplot(a, aes(x, y)) +
  geom_point() + stat_minus(colour = 'red')

GeomSimplePoint <- ggproto(
  'GeomSimplePoint',
  Geom,
  required_aes = c('x', 'y'),
  default_aes = aes(shape = 10, colour = 'black'),
  draw_key = draw_key_point,
  
  draw_panel = function(data, panel_params, coord) {
    coords <- coord$transform(data, panel_params)
    grid::pointsGrob(
      coords$x,
      coords$y,
      pch = coords$shape,
      gp = grid::gpar(col = coords$colour)
    )
  }
)

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


ggplot(mpg, aes(displ, hwy)) + geom_simple_point()