r - 从 ggplot2 对象中提取使用过的比例

Question

我想编写一个 ggplot2 主题，当 x 轴包含数值或因子时，它以不同的方式格式化 x 轴。

是否可以从主题调用中检测到使用了哪种类型的比例？如果是，如何？

我的代码看起来像这样，我正在寻找一个表达式来替换尖括号中的伪代码:

my_theme <- function(){
  thm <- theme_bw() %+replace%
    theme(
      panel.border = element_blank()
    )
  if(<x-Axis scale is factor?>){
    thm <- thm %+replace% 
             axis.ticks.x = element_blank()
  }
  thm
}

Answer 1

layer_scales 是 ggplot2 中的一个辅助函数，它返回与绘图的图层(默认情况下是第一个 geom 图层)关联的比例，所以类似于 class(layer_scales(plot)$ x) 可以告诉您正在处理的 x 轴类型。

这是一个如何实现它的例子:

# continuous x-axis plot (the additional specifications are there to make sure
# its look closely matches the next one
p1 <- ggplot(mtcars, aes(gear, wt, colour = factor(cyl))) +
  geom_point(size = 4, show.legend = FALSE) +
  scale_x_continuous(breaks = c(3, 4, 5),
                     expand = c(0, 0.6))

# discrete x-axis plot
p2 <- ggplot(mtcars, aes(factor(gear), wt, colour = factor(cyl))) +
  geom_point(size = 4, show.legend = FALSE)

my_theme <- function(plot){
  thm <- theme_bw() %+replace%
    theme(
      panel.border = element_blank()
    )
  if("ScaleDiscrete" %in% class(layer_scales(plot)$x)){
    thm <- thm %+replace% 
      theme(
        axis.ticks.x = element_blank()
      )
  }
  plot + thm
}

# check the difference in results for p1 & p2. p1 has axis ticks while p2 does not.
gridExtra::grid.arrange(my_theme(p1), my_theme(p2), nrow = 1)