如何减少ggplot中facet_wrap绘制的箱线图的箱体周边空白？

针对你遇到的分面箱线图左右空白冗余、箱体间距过大的问题，直接给你几个可落地的调整方案：

1. 砍掉x轴两侧多余空白 #

用scale_x_discrete的expand参数控制x轴的扩展空间，精准减少两侧空白：

gg_bio2 <- ggplot(tf2, aes(x=Paturage, y = log(Biomasse+1), fill = Paturage)) +
  geom_boxplot(width = 0.25)+
  scale_fill_manual(values = col) +
  scale_x_discrete(expand = c(0.1, 0))  # 第一个数值越小，两侧空白越少，可按需微调
  labs(x= "Condition de gestion", y="log(Biomasse+1) (g)") +
  facet_wrap(vars(Type.fonctionnel), drop = T, nrow = 8)+
  theme_bw()+
  theme(panel.grid = element_blank(),
        legend.position="none", 
        strip.background = element_rect(colour="black",
                                        fill="white"),
        axis.title.x = element_text(margin = unit(c(5, 0, 0, 0), "mm")),
        axis.title.y = element_text(margin = unit(c(0, 5, 0, 0), "mm")),
        text = element_text(size = 22))

2. 缩小两个箱体的横向间距 #

通过position_dodge调整箱体的位置间距，配合width参数优化箱体宽度，让两组箱体更紧凑：

gg_bio2 <- ggplot(tf2, aes(x=Paturage, y = log(Biomasse+1), fill = Paturage)) +
  geom_boxplot(width = 0.3, position = position_dodge(width = 0.4))  # dodge宽度小于箱体宽度时，间距会缩小
  scale_fill_manual(values = col) +
  scale_x_discrete(expand = c(0.1, 0))
  labs(x= "Condition de gestion", y="log(Biomasse+1) (g)") +
  facet_wrap(vars(Type.fonctionnel), drop = T, nrow = 8)+
  theme_bw()+
  theme(panel.grid = element_blank(),
        legend.position="none", 
        strip.background = element_rect(colour="black",
                                        fill="white"),
        axis.title.x = element_text(margin = unit(c(5, 0, 0, 0), "mm")),
        axis.title.y = element_text(margin = unit(c(0, 5, 0, 0), "mm")),
        text = element_text(size = 22))

3. 调整整体图尺寸避免过窄 #

如果调整后图体仍过窄，导出时用ggsave指定适配8行分面的宽高比例，保证文字和箱体清晰：

ggsave("biomasse_boxplot.pdf", gg_bio2, width = 10, height = 20, dpi = 300)

可根据实际显示效果微调width和height的数值。

内容的提问来源于stack exchange，提问作者Clara