如何为shadowtext::element_shadowtext设置背景色与半径参数

shadowtext包自带的element_shadowtext未开放bg.color、bg.r参数，是因为主题元素的编写逻辑没有把这两个参数透传给底层渲染，有两种成熟方案可以实现需求，不需要依赖不符合要求的其他包：

方案1：扩展原生element_shadowtext补全参数 #

直接重写element_shadowtext构造函数和对应的grob渲染方法，把缺失的两个参数加进去，用法和原生主题元素完全一致，不需要改动原有绘图逻辑：

# 重写element_shadowtext构造函数，新增bg.color、bg.r入参
element_shadowtext <- function(family = NULL, face = NULL, colour = NULL, size = NULL,
                               hjust = NULL, vjust = NULL, angle = NULL, lineheight = NULL,
                               color = NULL, margin = NULL, debug = NULL, inherit.blank = FALSE,
                               bg.colour = NULL, bg.color = NULL, bg.r = 0.1) {
  if (!is.null(color))  colour <- color
  if (!is.null(bg.color)) bg.colour <- bg.color
  structure(
    list(
      family = family, face = face, colour = colour, size = size,
      hjust = hjust, vjust = vjust, angle = angle, lineheight = lineheight,
      margin = margin, debug = debug, inherit.blank = inherit.blank,
      bg.colour = bg.colour, bg.r = bg.r
    ),
    class = c("element_shadowtext", "element_text", "element")
  )
}

# 重写对应渲染逻辑，透传背景参数到shadowtextGrob
element_grob.element_shadowtext <- function(element, label = "", x = NULL, y = NULL,
                                            family = NULL, face = NULL, colour = NULL, size = NULL,
                                            hjust = NULL, vjust = NULL, angle = NULL, lineheight = NULL,
                                            margin = NULL, margin_x = FALSE, margin_y = FALSE,
                                            bg.colour = NULL, bg.r = NULL, ...) {
  if (is.null(label)) return(ggplot2::zeroGrob())
  vj <- ggplot2:::calc_element("vjust", element, vjust)
  hj <- ggplot2:::calc_element("hjust", element, hjust)
  angle <- ggplot2:::calc_element("angle", element, angle)
  bg.colour <- if(is.null(bg.colour)) element$bg.colour else bg.colour
  bg.r <- if(is.null(bg.r)) element$bg.r else bg.r
  shadowtext::shadowtextGrob(
    label, x, y,
    default.units = "native",
    hjust = hj, vjust = vj, rot = angle,
    gp = grid::gpar(
      col = ggplot2:::calc_element("colour", element, colour),
      fontsize = ggplot2:::calc_element("size", element, size) * .pt,
      fontfamily = ggplot2:::calc_element("family", element, family),
      fontface = ggplot2:::calc_element("face", element, face),
      lineheight = ggplot2:::calc_element("lineheight", element, lineheight)
    ),
    bg.colour = bg.colour,
    bg.r = bg.r,
    ...
  )
}

使用时直接在主题配置里传参即可，和geom_shadowtext的参数逻辑完全一致：

data <- data.frame(ID= c("A", "B", "C"), Value =c(1.5,2,1.75))

library(ggplot2)
library(shadowtext)
ggplot(data, aes(x=ID, y=Value)) + 
  geom_col() + 
  theme(axis.text.x = element_shadowtext(
          color="black",
          bg.color = "white",
          bg.r = 0.2
        ),
        axis.title.x = element_blank())+
  geom_shadowtext(aes(label=Value), 
                  y=1, hjust=0.5,
                  bg.color="white", bg.r=0.2, color = "black")

方案2：手动绘制轴标签，零代码侵入 #

如果不想修改包的原生函数，可以直接隐藏默认坐标轴文本，用geom_shadowtext手动绘制轴标签，所有参数都可以直接复用，不需要额外写自定义函数：

ggplot(data, aes(x=ID, y=Value)) + 
  geom_col() + 
  # 隐藏默认x轴文本，预留底部边距
  theme(axis.text.x = element_blank(),
        axis.title.x = element_blank(),
        plot.margin = margin(b=15)) +
  # 柱形上的标注文本
  geom_shadowtext(aes(label=Value), 
                  y=1, hjust=0.5,
                  bg.color="white", bg.r=0.2, color = "black") +
  # 手动添加x轴标签，y值设为坐标轴下限以下的位置
  geom_shadowtext(aes(label=ID, y=-0.08),
                  bg.color="white", bg.r=0.2, color = "black") +
  # 关闭裁剪避免标签被截断，调整y轴范围适配
  coord_cartesian(ylim = c(0, 2.2), clip = "off")

这个方案的优点是不需要修改任何包内逻辑，所有shadowtext支持的参数都可以直接使用；缺点是需要根据图形尺寸手动调整标签的y轴位置和底部边距。

内容的提问来源于stack exchange，提问作者Beni