Adapt `draw_key()` according to own `draw_panel()` for new `ggproto`

Question

Based on the example in Master Software Development in R, I wrote a new geom_my_point(), adapting the alpha depending on the number of data points.

This works fine, but the alpha value of the label is not correct if alpha is explicitly set.

Here the code for the figures:

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")

Here the code for the 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)
  }
)

EDIT:

According to the comment of @teunbrand, the problem for the plot qq2 can be solved by the following adaptions to the draw_key() function:

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

But this still does not solve the problem with the graph qq3 - so the underlying question is why alpha is not correctly represented by the data argument of the draw_key() function. Compare also the following plot qq4, in which the size is correctly displayed in the legend (set a browser() w/i draw_key()):

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