如何通过标题而不是绘图区域来对齐多个绘图？

我在用着egg对齐页面上的多个绘图。我想知道是否可以通过标题对齐两列a) and c)而不是情节面积？谢谢！

Code:

library(egg)
library(grid)

p1 <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) +
  geom_point() + ggtitle("a)")
p1

p2 <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) +
  geom_point() + facet_wrap(~ cyl, ncol = 2, scales = "free") +
  guides(colour = "none") +
  theme() + ggtitle("b)")
p2

p3 <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) +
  geom_point() + facet_grid(. ~ am, scales = "free") + guides(colour="none") +
  ggtitle("c)")
p3

g1 <- ggplotGrob(p1)

g2 <- ggplotGrob(p2)

g3 <- ggplotGrob(p3)

fg1 <- gtable_frame(g1, debug = TRUE)
fg2 <- gtable_frame(g2, debug = TRUE)
fg12 <- gtable_frame(gtable_rbind(fg1, fg2),
                     width = unit(2, "null"),
                     height = unit(1, "null"))
fg3 <-
  gtable_frame(
    g3,
    width = unit(2, "null"),
    height = unit(1, "null"),
    debug = TRUE
  )
grid.newpage()
combined <- gtable_cbind(fg12, fg3)
grid.draw(combined)  

Plot:

我通过使用找到了另一种方法cowplot包裹

left_col <- cowplot::plot_grid(p1 + ggtitle(""), p2 + ggtitle(""), 
                               labels = c('a)', 'b)'), label_size = 14,
                               ncol = 1, align = 'v', axis = 'lr') 
cowplot::plot_grid(left_col, p3 + ggtitle(""), 
                   labels = c('', 'c)'), label_size = 14,
                   align = 'h', axis = 'b')

也可以看看here

Edit:

最近开发的一个包patchwork for ggplot2也能完成工作

library(patchwork)

{
  p1 + {p2} + patchwork::plot_layout(ncol = 1)
} / p3 + patchwork::plot_layout(ncol = 2)