我使用grid：：segmentsGrob（arrow）向ggplot添加箭头。我用x轴坐标定义箭头的位置。出于某种原因，箭头的高度根据箭头的长度进行缩放。我希望箭头占据相同的高度，只缩短箭头的长度。我是否缺少了一个高度参数？

library(ggplot2)
library(grid)
df <- data.frame(
  Position=(c(1:5000)),
  Signal=(c((rep(c(5),times=2000)), (rep(c(100),times=1000)), (rep(c(5),times=2000))))
  )
Plotv1 <- ggplot()+
  geom_line(data = df, aes(x=Position, y=Signal, col = "#000000"))+
  coord_cartesian(clip="off") +
  theme(axis.text.x = element_blank()) +
  theme(
    axis.title.x = element_text(margin=margin(t=30)),
    legend.title = element_text(colour = "#000000", size=12),
    legend.text = element_text(colour = "#000000", size=12)
  ) +
  guides(fill = "none") +
  annotation_custom(
    grid::segmentsGrob(
      y0 = unit(-0.3, "npc"),
      y1 = unit(-0.3, "npc"),
      arrow = arrow(angle=45, length = unit(.15, 'npc')),
      gp = grid::gpar(lwd=3, fill = "#000000")
    ),
    xmin = 1,
    xmax = 1700
  ) +
  annotation_custom(
    grid::segmentsGrob(
      y0 = unit(-0.3, "npc"),
      y1 = unit(-0.3, "npc"),
      arrow = arrow(angle=45, length = unit(.15, 'npc')),
      gp = grid::gpar(lwd=3, fill = "#000000")
    ),
    xmin = 2000,
    xmax = 2200
  ) +
  annotation_custom(
    grid::segmentsGrob(
      y0 = unit(-0.3, "npc"),
      y1 = unit(-0.3, "npc"),
      arrow = arrow(angle=45, length = unit(.15, 'npc')),
      gp = grid::gpar(lwd=3, fill = "#000000")
    ),
    xmin = 2500,
    xmax = 5000
  )
ggsave(paste("~/Desktop/Plotv1.png", sep = ""), Plotv1, width = 8, height = 1.7)