R语言geom_arc_bar制作分离饼图异常：explode参数问题求助

我尝试用R语言制作各扇区均匀分离的饼图，原本计划通过ggforce包的geom_arc_bar()结合ggplot的explode参数实现，但结果出现异常。

问题复现：全局设置explode后分离不均 #

运行以下代码后，得到的饼图扇区分离不均匀：

library(ggforce)
library(ggplot2)

pie <- data.frame( days_in_each_month = c(31,  28,  31,  30,  31,  30,  31,  31,  30,  31,  30,  31),
                    months = c("01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"))

p <- ggplot() + theme_no_axes() + coord_fixed() + theme(legend.position="none")
#define the inner set of background wedges
p <- p + geom_arc_bar(data = pie, stat = 'pie', aes(x0 = 0, y0 = 0, r0 = .1, r = .5, amount = days_in_each_month, fill = months, explode = 0.02))

p

部分设置为0时效果改善 #

经过测试发现，若为每个扇区单独指定explode参数，且最后三个扇区设为0时，分离效果有所改善：

pie <- data.frame( days_in_each_month = c(31,  28,  31,  30,  31,  30,  31,  31,  30,  31,  30,  31),
                   months = c("01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"))

expl <- c(0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0, 0, 0)

p <- ggplot() + theme_no_axes() + coord_fixed() + theme(legend.position="none")
#define the inner set of background wedges
p <- p + geom_arc_bar(data = pie, stat = 'pie', aes(x0 = 0, y0 = 0, r0 = .1, r = .5, amount = days_in_each_month, fill = months, explode = expl))

p

修改非零值后再次出现不均 #

但如果将expl中的任意一个0改为其他值（比如最后一个设为0.2），饼图的分离效果又会变得不均匀：

pie <- data.frame( days_in_each_month = c(31,  28,  31,  30,  31,  30,  31,  31,  30,  31,  30,  31),
                   months = c("01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"))

expl <- c(0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0.02, 0, 0, 0.2)

p <- ggplot() + theme_no_axes() + coord_fixed() + theme(legend.position="none")
#define the inner set of background wedges
p <- p + geom_arc_bar(data = pie, stat = 'pie', aes(x0 = 0, y0 = 0, r0 = .1, r = .5, amount = days_in_each_month, fill = months, explode = expl))

p

我无法解释这种现象，请问这是什么原因？

内容的提问来源于stack exchange，提问作者birdy448