如何将gratia::compare_smooths生成的图例强制置于底部？

问题原因 #

gratia的draw()方法默认使用theme_gratia()主题，该主题默认将图例置于右侧。直接叠加theme(legend.position = "bottom")可能因主题元素优先级问题未生效，需要明确覆盖主题设置或在draw()中直接指定自定义主题。

解决方案 #

方案1：在draw()中直接指定自定义主题

在调用draw()时，直接传入包含图例位置设置的主题，确保样式生效：

library(tidyverse)
library(gratia)
library(mgcv)
library(ggpubr)

gam1 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl >= 6))
gam2 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl < 6))

gam3 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec >= 18))
gam4 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec < 18))

# 在draw中指定主题，设置图例在底部并横向排列
plot1 <- draw(compare_smooths(gam1,gam2),
              theme = theme_gratia() + theme(legend.position = "bottom",
                                             legend.direction = "horizontal"))
plot2 <- draw(compare_smooths(gam3,gam4),
              theme = theme_gratia() + theme(legend.position = "bottom",
                                             legend.direction = "horizontal"))

gamplot <- ggarrange(plotlist = list(plot1,plot2),ncol = 2) 
gamplot

方案2：全局设置主题（所有绘图自动应用）

通过theme_set()全局修改gratia主题的图例位置，后续所有draw()生成的图都会自动应用该设置：

library(tidyverse)
library(gratia)
library(mgcv)
library(ggpubr)

# 全局修改gratia主题的图例位置
theme_set(theme_gratia() + theme(legend.position = "bottom",
                                 legend.direction = "horizontal"))

gam1 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl >= 6))
gam2 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl < 6))

gam3 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec >= 18))
gam4 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec < 18))

plot1 <- draw(compare_smooths(gam1,gam2))
plot2 <- draw(compare_smooths(gam3,gam4))

gamplot <- ggarrange(plotlist = list(plot1,plot2),ncol = 2) 
gamplot

方案3：使用主题替换语法强制覆盖

若直接叠加主题不生效，可使用%+replace%强制替换主题元素：

library(tidyverse)
library(gratia)
library(mgcv)
library(ggpubr)

gam1 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl >= 6))
gam2 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(cyl < 6))

gam3 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec >= 18))
gam4 <- gam(mpg ~ am + s(hp),data = mtcars |> filter(qsec < 18))

plot1 <- draw(compare_smooths(gam1,gam2)) +
  theme_gratia() %+replace% theme(legend.position = "bottom",
                                  legend.direction = "horizontal")
plot2 <- draw(compare_smooths(gam3,gam4)) +
  theme_gratia() %+replace% theme(legend.position = "bottom",
                                  legend.direction = "horizontal")

gamplot <- ggarrange(plotlist = list(plot1,plot2),ncol = 2) 
gamplot

补充说明 #

compare_smooths仅负责生成平滑项的对比数据，不处理绘图样式，因此图例位置的调整需要通过draw()的主题参数或后续ggplot主题设置来实现。

内容的提问来源于stack exchange，提问作者flâneur