r - 根据自己的`draw_panel()`为新的`ggproto`适配`draw_key()`

Question

基于Master Software Development in R中的示例，我编写了一个新的，根据数据点的数量进行geom_my_point()调整。alpha

这工作正常，但如果明确设置了 alpha，则标签的 alpha 值不正确。

这里是数字的代码：

d <- data.frame(x = runif(200))
d$y <- 1 * d$x + rnorm(200, 0, 0.2)
d$z <- factor(sample(c("group1", "group2"), size = 200, replace = TRUE))

require("ggplot2")
gg1 <- ggplot(d) + geom_my_point(aes(x, y, colour = z)) + ggtitle("gg1")
gg2 <- ggplot(d) + geom_my_point(aes(x, y, colour = z), alpha = 1) + ggtitle("gg2")
gg3 <- ggplot(d) + geom_my_point(aes(x, y, colour = z, alpha = z)) + ggtitle("gg3")

这里的代码geom_*()：

geom_my_point <- function(mapping = NULL, data = NULL, stat = "identity",
                            position = "identity", na.rm = FALSE,
                            show.legend = NA, inherit.aes = TRUE, ...) {
  ggplot2::layer(
    geom = GeomMyPoint, mapping = mapping,
    data = data, stat = stat, position = position,
    show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
}


GeomMyPoint <- ggplot2::ggproto("GeomMyPoint", ggplot2::Geom,
  required_aes = c("x", "y"),
  non_missing_aes = c("size", "shape", "colour"),
  default_aes = ggplot2::aes(
    shape = 19, colour = "black", size = 2,
    fill = NA, alpha = NA, stroke = 0.5
  ),

  setup_params = function(data, params) {
    n <- nrow(data)
    if (n > 100 && n <= 200) {
      params$alpha <- 0.3
    } else if (n > 200) {
      params$alpha <- 0.15
    } else {
      params$alpha <- 1
    }
    params
  },

  draw_panel = function(data, panel_scales, coord, alpha) {
    if (is.character(data$shape)) {
      data$shape <- translate_shape_string(data$shape)
    }

    ## Transform the data first
    coords <- coord$transform(data, panel_scales)

    ## Get alpha conditional on number of data points
    if (any(is.na(coords$alpha))) {
      coords$alpha <- alpha
    }

    ## Construct a grid grob
    grid::pointsGrob(
      x = coords$x,
      y = coords$y,
      pch = coords$shape,
      gp = grid::gpar(
        col = alpha(coords$colour, coords$alpha),
        fill = alpha(coords$fill, coords$alpha),
        fontsize = coords$size * ggplot2::.pt + coords$stroke * ggplot2::.stroke / 2,
        lwd = coords$stroke * ggplot2::.stroke / 2
      )
    )
  },

  draw_key = function(data, params, size) {
    data$alpha <- params$alpha
    ggplot2::draw_key_point(data, params, size)
  }
)

编辑：

根据@teunbrand 的评论，情节qq2 的问题可以通过对draw_key()函数的以下改编来解决：

draw_key = function(data, params, size) { 
  if (is.na(data$alpha)) { 
    data$alpha <- params$alpha
  } 
  ggplot2::draw_key_point(data, params, size)
}

但这仍然不能解决图 qq3 的问题 - 所以根本问题是为什么函数的参数alpha没有正确表示。还比较下图 qq4，其中正确显示在图例中（设置w/i ）：datadraw_key()sizebrowser()draw_key()

gg4 <- ggplot(d) + geom_my_point(aes(x, y, colour = z, alpha = z, size = z)) + ggtitle("gg4")