cockplot：如何通过修改轴限制来自定义主面板和边缘图之间的间隙？

这是@ClausWilke 提供的解决方案之一的后续问题（see post https://stackoverflow.com/questions/47542849/marginal-plots-using-axis-canvas-in-cowplot-how-to-insert-gap-between-main-pane）在主面板和边缘图之间插入间隙。 如何确定 (scale_x_连续) 限制？ 另外，如果我们使用“NA”作为上限会发生什么？

# Example with limits set to: (-2,4.5)
require(ggplot2)
require(cowplot)

pmain <- ggplot(data = mpg, aes(x = cty, y = hwy)) + 
  geom_point() + 
  xlab("City driving (miles/gallon)") +
  ylab("Highway driving (miles/gallon)") + 
  theme_grey()

xbox2 <- axis_canvas(pmain, axis = "x", coord_flip = TRUE) + 
  geom_boxplot(data = mpg, aes(y = cty, x = 1))  + 
  scale_x_continuous(limits = c(-2, 4.5)) + coord_flip()

ybox2 <- axis_canvas(pmain, axis = "y") + 
  geom_boxplot(data = mpg, aes(y = hwy, x = 1)) + 
  scale_x_continuous(limits = c(-2, 4.5))

p1 <- insert_xaxis_grob(pmain, xbox2, grid::unit(0.8, "in"), position = "top")
p2 <- insert_yaxis_grob(p1, ybox2, grid::unit(0.8, "in"), position = "right")

ggdraw(p2)

对于连续的 x 轴刻度，您可以使用expand_limits在主图和边缘图之间添加一些空白。

例如，您的ybox2是具有连续 x 轴刻度的单箱形图。默认情况下，该图的 x 范围大约为 0.6 到 1.4（您可以通过运行来查看这一点）ggplot(mpg, aes(y=hwy, x=1)) + geom_boxplot()）。以下代码将 x 轴下限设置为 0.2，这意味着 x 轴范围的大约 0.4/(0.8+0.4) = 33% 将是主图和边缘图之间的边距。我们也做同样的事情xbox2.

ybox2 <- axis_canvas(pmain, axis = "y") + 
  geom_boxplot(data = mpg, aes(y = hwy, x = 1)) +
  expand_limits(x = 0.2)

xbox2 <- axis_canvas(pmain, axis = "x", coord_flip = TRUE) + 
  geom_boxplot(data = mpg, aes(y = cty, x = 1))  + 
  coord_flip() +
  expand_limits(x = 0.2)

p1 <- insert_xaxis_grob(pmain, xbox2, grid::unit(0.4, "in"), position = "top")
p2 <- insert_yaxis_grob(p1, ybox2, grid::unit(0.4, "in"), position = "right")

ggdraw(p2)