R网格：＆＃39; grob＆＃39;路径必须包含至少一个名称＆＃39; grob＆＃39;

Question

我想在ggplot2图表中添加箭头。我不断收到错误：＆＃39; grob＆＃39;路径必须包含至少一个名称＆＃39; grob＆＃39;。

library(ggplot2)
library(grid)

#create a plot
grid.newpage()
update_geom_defaults("smooth", aes(color="black"))
print(
  ggplot(mtcars, aes(x=disp, y=mpg)) +
    geom_point() +
    geom_smooth(method=lm)

)

#list views
current.vpTree()

# change the view
downViewport("panel.6-4-6-4")

# add an arrow
sline <- grid.get(gPath("smooth", "polyline"),
                  grep=TRUE)
grid.segments(.7, .8, 
              grobX(sline, 45), grobY(sline, 45),
              arrow=arrow(angle=10, type="closed"),
              gp=gpar(fill="black"))