将彩色箭头添加到ggplot2的轴(部分在绘图区域外)

Question

我想添加一个彩色箭头(轴的全长)来显示向一个方向移动的时间(这可以假设,但是对于这个图,没有数值,所以我希望箭头显示方向).我可以geom_segment用来绘制它,但是缺少了绘图区域之外的部分.

我看过这篇文章:R&ggplot2:如何在轴标签下获得箭头？但这个解决方案是轴标题的黑客.这篇文章:https://stackoverflow.com/a/10542622/1000343显示文本区域之外的行,但不是彩色箭头.

MWE

library(ggplot2); library(grid); library(scales)

dat <- data.frame(Time=0:5, y=0:5)

ggplot(dat, aes(x=Time, y=y)) +
    geom_area(alpha=.1) + theme_bw() +
    scale_y_continuous(expand = c(0, 0)) +
    scale_x_continuous(expand = c(0, 0)) +
    theme(panel.grid.major = element_blank(), 
        panel.grid.minor = element_blank(),
          axis.text.x=element_blank(),
          axis.ticks.x=element_blank()
    )    

我试过了:

ggplot(dat, aes(x=Time, y=y)) +
    geom_area(alpha=.1) + theme_bw() +
    scale_y_continuous(expand = c(0, 0)) +
    scale_x_continuous(expand = c(0, 0)) +
    theme(panel.grid.major = element_blank(), 
        panel.grid.minor = element_blank(),
          axis.text.x=element_blank(),
          axis.ticks.x=element_blank()
    ) +
    geom_segment(aes(x=0, xend = 5 , y=0, yend = 0), size=1.5,
        arrow = arrow(length = unit(0.6,"cm"))) 

给予

但我想要

Answer 1

你可以定义自己的轴grob,

library(ggplot2)

element_grob.element_custom <- function(element, ...)  {
  grid::segmentsGrob(0,1,1,1, arrow = arrow())
}
 ## silly wrapper to fool ggplot2
axis_custom <- function(...){
  structure(
    list(...), # this ... information is not used, btw
    class = c("element_custom","element_blank", "element") # inheritance test workaround
  ) 

}

ggplot(iris, aes(Sepal.Length, Sepal.Width)) +
  geom_line() +
  theme(axis.line = axis_custom(),
        axis.line.y=element_blank())

Answer 2

问题似乎只是裁剪区域(如此处所述).尝试:

p1<-ggplot(dat, aes(x=Time, y=y)) +
    geom_area(alpha=.1) + theme_bw() +
    scale_y_continuous(expand = c(0, 0)) +
    scale_x_continuous(expand = c(0, 0)) +
    theme(panel.grid.major = element_blank(), 
        panel.grid.minor = element_blank(),
          axis.text.x=element_blank(),
          axis.ticks.x=element_blank()
    ) +
    geom_segment(aes(x=0, xend = 5 , y=0, yend = 0), size=1.5,
        arrow = arrow(length = unit(0.6,"cm"))) 

gt <- ggplot_gtable(ggplot_build(p1))
gt$layout$clip[gt$layout$name=="panel"] <- "off"
grid.draw(gt)

要得到