r - 向 R 中的刻面图添加刻度线

Question

我希望在刻面图中添加刻度线。
例如，对于这样的分面图:

require(ggplot2)

p <- ggplot(mpg, aes(displ, hwy)) + 
     geom_point() + 
     theme_minimal() +
     geom_hline(yintercept=10, linetype="solid") +
     geom_vline(xintercept=1.0, linetype="solid") +
     scale_x_continuous(limits=c(1,8), expand =c(0,0), 
     breaks = rep(2:7)) +
     scale_y_continuous(limits = c(10, 50), expand = c(0,0), 
     breaks = c(20,30,40)) +
     facet_wrap(~cyl) +   
     theme(panel.margin.x=unit(1.2, "lines"),
     panel.margin.y=unit(1.2, "lines"))
p

我希望添加刻度线，使其看起来像这样(请注意，虽然刻度线已添加到所有方面，但刻度标签仅显示在左侧和底部方面):



我试过使用 annotation_custom向轴添加线条格子的函数。但是我的尝试不起作用。

require(gridExtra)
require(grid)   
x.breaks = rep(2:7)
y.breaks = c(20,30,40)

for (i in 1:length(x.breaks))   {
  p = p + annotation_custom(grob = linesGrob(y = unit(c(0, 5), "npc"),
                       gp=gpar(col= "black")),  
                       xmin = x.breaks[i], 
                       xmax = x.breaks[i], 
                       ymin = 8.69, 
                       ymax = 8.89)
  }

for (i in 1:length(y.breaks))   {
  p = p + annotation_custom(grob = linesGrob(x = unit(c(0, 5), "npc"),
                       gp=gpar(col= "black")),  
                       xmin = 0.769,
                       xmax = 0.809,                           
                       ymin = y.breaks[i], 
                       ymax = y.breaks[i])
  }

gt = ggplotGrob(p)
gt$layout$clip[gt$layout$name=="panel"] <- "off"
grid.draw(gt)

有人有什么建议吗？提前致谢。

Answer 1

这是一种削减数字的低级方法，

p <- ggplot(mpg, aes(displ, hwy)) + 
  geom_point() + 
  theme_bw() +
  geom_hline(yintercept=10, linetype="solid") +
  geom_vline(xintercept=1.0, linetype="solid") +
  scale_x_continuous(limits=c(1,8), expand =c(0,0), 
                     breaks = rep(2:7)) +
  scale_y_continuous(limits = c(10, 50), expand = c(0,0), 
                     breaks = c(20,30,40)) +
  facet_wrap(~cyl, scales="free") 

g <- ggplotGrob(p)
g$grobs$axis_b1$children[[2]][["grobs"]][[2]] <- nullGrob()
g$grobs$axis_b2$children[[2]][["grobs"]][[2]] <- nullGrob()
grid.newpage()
grid.draw(g)

还可以删除gtable中的相应空间。