ggplot2自定义Geom线段图例符号显示异常问题求助

背景 #

我正在R中基于geom_segment()开发ggplot2的自定义geom，代码如下：

geom_custom <- function(mapping = NULL, data = NULL,
                     stat = "identity", position = "identity",
                     ..., na.rm = FALSE, show.legend = NA,
                     inherit.aes = TRUE) {
  
  out <- layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = CustomGeom,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
  
  out
}

CustomGeom <- ggproto("CustomGeom", Geom,
                    required_aes = c("x", "y", "xend"),
                    non_missing_aes = c("linetype", "linewidth"),
                    default_aes = aes(
                      colour = "black",
                      linewidth = 2,
                      size = 2,
                      linetype = 1,
                      alpha = NA
                    ),
                    draw_panel = function(data, panel_params, coord, ...) {
                      
                      # Return all components
                      grid::gList(
                        GeomSegment$draw_panel(data, panel_params, coord, ...)
                      )
                    }
)

问题 #

当前问题是图例符号错误地显示为形状，而非geom_segment()默认的线段。以下是示例可复现代码：

df <- tibble::tribble(
  ~"record", ~"status", ~"start_time", ~"end_time",
  1, "status1", 0, 5,
  2, "status1", -2, 7,
  3, "status2", 2, 15
)

ggplot() +
  geom_custom(data = df,
              mapping = aes(x = start_time, xend = end_time, y = record, colour = status))

当前输出图例为形状，而使用geom_segment()的预期输出图例为线段。

解决方案 #

问题根源在于自定义的CustomGeom继承自基础Geom类，其默认的图例绘制函数是用于点形状的draw_key_point，而geom_segment()使用的是线段类型的draw_key_path。你可以通过以下两种方式修正：

方法1：让自定义Geom继承自GeomSegment #

既然你的自定义geom本质上调用的是GeomSegment的绘制逻辑，直接让CustomGeom继承GeomSegment，就能自动继承正确的图例绘制规则，同时简化代码：

geom_custom <- function(mapping = NULL, data = NULL,
                       stat = "identity", position = "identity",
                       ..., na.rm = FALSE, show.legend = NA,
                       inherit.aes = TRUE) {
  
  out <- layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = CustomGeom,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
  
  out
}

CustomGeom <- ggproto("CustomGeom", GeomSegment,
                     required_aes = c("x", "y", "xend"), # 保留自定义的必填美学映射
                     non_missing_aes = c("linetype", "linewidth"),
                     default_aes = aes(
                       colour = "black",
                       linewidth = 2,
                       size = 2,
                       linetype = 1,
                       alpha = NA
                     ),
                     draw_panel = function(data, panel_params, coord, ...) {
                       GeomSegment$draw_panel(data, panel_params, coord, ...)
                     }
)

方法2：显式指定图例绘制函数 #

如果需要保持继承自Geom，只需在CustomGeom中添加draw_key = draw_key_path参数，指定用线段绘制图例：

CustomGeom <- ggproto("CustomGeom", Geom,
                    required_aes = c("x", "y", "xend"),
                    non_missing_aes = c("linetype", "linewidth"),
                    default_aes = aes(
                      colour = "black",
                      linewidth = 2,
                      size = 2,
                      linetype = 1,
                      alpha = NA
                    ),
                    draw_key = draw_key_path, # 添加该行指定图例绘制函数
                    draw_panel = function(data, panel_params, coord, ...) {
                      
                      grid::gList(
                        GeomSegment$draw_panel(data, panel_params, coord, ...)
                      )
                    }
)

两种方法都能让图例显示为geom_segment()默认的线段样式，运行示例代码即可验证效果。

内容的提问来源于stack exchange，提问作者rsh52