使用ggdraw/plot_grid/ggsave添加多图主X轴标题时JPG丢失标题

问题描述 #

我用cowplot包创建2×4的多图面板，通过ggdraw()和draw_label()制作跨图的主X轴标题，再用plot_grid()把标题放在两行图形下方。简化代码如下：

# 创建轴标题对象
xTitle = ggdraw() +
           draw_label("lorem ipsum",
                      fontface = "bold",
                      hjust = 0.5) +
           theme(plot.margin = margin(0,0,0,7))

# 创建图形
ggplot(data = iris,
       mapping = aes(x = Sepal.Width,
                     y = Sepal.Length)) +
       geom_point() -> p1

ggplot(data = iris,
       mapping = aes(x = Sepal.Width,
                     y = Sepal.Length)) +
       geom_point() -> p2

# 创建图形网格
grid <- cowplot::plot_grid(p1, p2, xTitle, ncol = 1, rel_heights = c(1,1,0.2))

在RStudio中预览或用ggsave()保存为PDF时显示正常，但保存为JPG时，底部的主X轴标题会被遗漏。需要解决这个问题，或指出操作错误、提供替代方案。

会话信息：

R version 4.2.2 (2022-10-31 ucrt)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 22621)

Matrix products: default

locale:
[1] LC_COLLATE=English_Australia.utf8  LC_CTYPE=English_Australia.utf8   
[3] LC_MONETARY=English_Australia.utf8 LC_NUMERIC=C                       
[5] LC_TIME=English_Australia.utf8    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] multcomp_1.4-20 TH.data_1.1-1   MASS_7.3-58.1   survival_3.4-0  mvtnorm_1.1-3  
 [6] cowplot_1.1.1   emmeans_1.8.0   nlme_3.1-159    forcats_0.5.2   stringr_1.4.1  
[11] dplyr_1.0.10    purrr_0.3.4     readr_2.1.2     tidyr_1.2.0     tibble_3.1.8   
[16] ggplot2_3.3.6   tidyverse_1.3.2

loaded via a namespace (and not attached):
 [1] lubridate_1.8.0     lattice_0.20-45     zoo_1.8-10          digest_0.6.29      
 [5] assertthat_0.2.1    utf8_1.2.2          R6_2.5.1            cellranger_1.1.0   
 [9] backports_1.4.1     reprex_2.0.2        coda_0.19-4         httr_1.4.4         
[13] pillar_1.8.1        rlang_1.0.6         googlesheets4_1.0.1 readxl_1.4.1       
[17] rstudioapi_0.14     Matrix_1.4-1        labeling_0.4.2      textshaping_0.3.6  
[21] splines_4.2.2       googledrive_2.0.0   munsell_0.5.0       broom_1.0.1        
[25] compiler_4.2.2      modelr_0.1.9        systemfonts_1.0.4   pkgconfig_2.0.3    
[29] tidyselect_1.2.0    codetools_0.2-18    fansi_1.0.3         crayon_1.5.1       
[33] tzdb_0.3.0          dbplyr_2.2.1        withr_2.5.0         grid_4.2.2         
[37] jsonlite_1.8.0      xtable_1.8-4        gtable_0.3.1        lifecycle_1.0.3    
[41] DBI_1.1.3           magrittr_2.0.3      scales_1.2.1        estimability_1.4.1 
[45] cli_3.3.0           stringi_1.7.8       farver_2.1.1        fs_1.5.2           
[49] xml2_1.3.3          ragg_1.2.2          ellipsis_0.3.2      generics_0.1.3     
[53] vctrs_0.4.1         sandwich_3.0-2      tools_4.2.2         glue_1.6.2         
[57] hms_1.1.2           colorspace_2.0-3    gargle_1.2.0        rvest_1.0.3        
[61] haven_2.5.1 

解决方案 #

方法1：调整标题边距并指定稳定渲染设备 #

问题根源是标题对象的负边距导致JPG裁剪时丢失内容，同时使用ragg设备能提升文本渲染稳定性：

# 调整标题边距，避免负边距引发裁剪
xTitle = ggdraw() +
           draw_label("lorem ipsum",
                      fontface = "bold",
                      hjust = 0.5) +
           theme(plot.margin = margin(5, 0, 5, 0)) # 上下保留少量边距

# 生成图形网格
grid <- cowplot::plot_grid(p1, p2, xTitle, ncol = 1, rel_heights = c(1,1,0.1))

# 使用ragg设备保存JPG
ggsave("plot.jpg", grid, device = ragg::agg_jpeg, width = 8, height = 6, dpi = 300)

方法2：利用plot_grid()的bottom参数直接添加标题 #

cowplot的plot_grid()支持通过bottom参数直接挂载底部标题，无需单独创建标题对象，能规避裁剪问题：

# 先组合两行图形
row_plots <- cowplot::plot_grid(p1, p2, ncol = 1, rel_heights = c(1,1))

# 添加底部主X轴标题
grid <- cowplot::plot_grid(row_plots, 
                           bottom = ggdraw() + draw_label("lorem ipsum", fontface = "bold"))

# 保存JPG
ggsave("plot.jpg", grid, width = 8, height = 6, dpi = 300)

方法3：改用patchwork包实现多图布局 #

patchwork对跨图标题的支持更直观，渲染稳定性优于当前方案：

library(patchwork)

# 组合图形并添加底部标题
grid <- (p1 / p2) + plot_annotation(caption = "lorem ipsum", 
                                    theme = theme(plot.caption = element_text(hjust = 0.5, fontface = "bold", size = 12)))

# 保存JPG
ggsave("plot.jpg", grid, width = 8, height = 6, dpi = 300)

内容的提问来源于stack exchange，提问作者llewmills