ggplot：coord_cartesian（clip =“ off”）和alpha <1.0隐藏geom_lines

Question

在ggplot中，当关闭裁剪时，alpha <1.0的行将被完全省略。

这是一个例子：

## create a dummy data frame:
library(tidyverse)

num_lines <- 10

start_values = runif(num_lines, min = 0, max = 5)
end_values = start_values + runif(num_lines, min = 0, max = 7)

df <- data.frame(from = start_values, to = end_values, 
                 value = runif(num_lines, min = 0, max = 100),
                 name = letters[seq(1, num_lines)])

plot_df <- df %>% gather(milestone, date, -name, - value)

## draw it:
ggplot(plot_df, aes(x = date, y = name)) + 
    geom_line(aes(size = value), color = "grey50", alpha = 0.9) + #### lines are not drawn
    geom_line(linetype = "dashed", alpha = 1.0) +                 #### lines are drawn
    annotate("label", x = num_lines/2, y = num_lines + 1, label = "A label") + #### a label that should not be clipped...
    coord_cartesian(clip = "off") +                                            #### ...therefore: clip = "off"
    ggtitle("Example")

在运行代码时，未绘制第一个（... alpha = 0.9）的geom_line，而第二个的行为符合预期。删除coord_cartesian(clip = "off")时，两个geom_lines都会出现。

这是ggplot中的错误还是我做错了什么？