在ggplot中，当关闭裁剪时，alpha

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,max = 7)

df <- data.frame(from = start_values,to = end_values,value = runif(num_lines,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？