ggplot2/grid：无论长度如何，保持分段箭头相同的比例

Question

我使用 grid::segmentsGrob(arrow) 将箭头添加到我的 ggplot 中。我用 x 轴坐标定义箭头的位置。由于某种原因，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，而只是缩短箭头的长度。我是否缺少高度参数？

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)

Answer 1

问题是您以

"npc"

单位（又称标准化父坐标）指定了箭头的长度。因此，随着父视口的绝对宽度和高度不同，箭头的长度也会有所不同，即在您的情况下，每个箭头的视口是由

xmin

和

xmax

定义的矩形。相反，请对箭头的

length

使用绝对单位。此外，我建议也使用绝对单位来定位箭头。

library(ggplot2)
library(grid)

df <- data.frame(
  Position = 1:5000,
  Signal = c(
    rep(5, times = 2000),
    rep(100, times = 1000),
    rep(5, times = 2000)
  )
)

segment <- grid::segmentsGrob(
  y0 = unit(-10, "pt"),
  y1 = unit(-10, "pt"),
  arrow = arrow(angle = 45, length = unit(.15, "cm")),
  gp = grid::gpar(lwd = 3, fill = "#000000")
)

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(
    segment, xmin = 1, xmax = 1700
  ) +
  annotation_custom(
    segment, xmin = 2000, xmax = 2200
  ) +
  annotation_custom(
    segment, xmin = 2500, xmax = 5000
  )