ggplot2次坐标轴出现多条网格线：原因排查与解决方法

问题描述 #

使用以下代码绘图时，图表在0、20.31等位置出现多条重复的水平网格线：

library(ggplot2)
ggplot (structure(list(Group1 = c('A','B', 'B','A', 'B','B', 'A', 'B', 'B'), Val = c(40.707, -22.513, -3.501, -12.884, -19.668,
-5.976, -16.721, -15.838, -5.59)), row.names = c(NA, -9L), class = "data.frame"), aes (x = Group1, y = Val)) +
geom_line () +
scale_y_continuous (breaks = c(0, -22.51, 20.31, 41.72, 63.13, 84.54) ,
sec.axis = sec_axis(~.,
breaks = c(0, -22.51, 20.31, 41.72, 63.13, 84.54)))

原因分析 #

重复网格线的根源是：你同时给主Y轴和次Y轴设置了完全一致的breaks参数。ggplot2会为每个轴独立生成对应刻度的网格线，当两个轴的刻度位置完全重合时，两条网格线叠在一起，视觉上就呈现出“重复”的效果。

解决方法 #

只需要在次轴的配置中关闭网格线的生成即可，通过guide = guide_axis(theme = theme(axis.grid = element_blank()))来隐藏次轴的网格线。

修改后的代码 #

library(ggplot2)
ggplot (structure(list(Group1 = c('A','B', 'B','A', 'B','B', 'A', 'B', 'B'), Val = c(40.707, -22.513, -3.501, -12.884, -19.668,
-5.976, -16.721, -15.838, -5.59)), row.names = c(NA, -9L), class = "data.frame"), aes (x = Group1, y = Val)) +
geom_line () +
scale_y_continuous (
  breaks = c(0, -22.51, 20.31, 41.72, 63.13, 84.54),
  sec.axis = sec_axis(
    ~.,
    breaks = c(0, -22.51, 20.31, 41.72, 63.13, 84.54),
    guide = guide_axis(theme = theme(axis.grid = element_blank()))
  )
)

简化版代码（可选） #

如果次轴的刻度和主轴完全一致，其实没必要重复设置breaks，可以简化成这样：

library(ggplot2)
ggplot (structure(list(Group1 = c('A','B', 'B','A', 'B','B', 'A', 'B', 'B'), Val = c(40.707, -22.513, -3.501, -12.884, -19.668,
-5.976, -16.721, -15.838, -5.59)), row.names = c(NA, -9L), class = "data.frame"), aes (x = Group1, y = Val)) +
geom_line () +
scale_y_continuous (
  breaks = c(0, -22.51, 20.31, 41.72, 63.13, 84.54),
  sec.axis = sec_axis(~., guide = guide_axis(theme = theme(axis.grid = element_blank())))
)

内容的提问来源于stack exchange，提问作者Daniel Lobo