如何向多个 ggplot 图表添加标签（A、B、C）

Question

我试图将标签 A、B 和 C 添加到每个图表的左上角。我已经尝试过cowplot::draw_plot_label()，但似乎没有任何效果。有人可以帮忙吗？

这些 A、B 和 C 标签并不是每个图的主标题。

# Packages
library(ggplot2)
library(gridExtra)
library(cowplot)

# 1st plot
p1 <- ggplot(data = new_data %>% 
               filter(Species =="Sharksucker_Remora")) +
  scale_colour_manual(values=c(Sharksucker_Remora="black"), labels = c("Sharksucker Remora")) + 
  geom_line(mapping = aes(x = Date, y = Proportion, group = Species, colour = Species)) + 
  xlab("") + 
  ylab("Proportion") + 
  theme(legend.position="top") +
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1)) + labs(colour = ~italic(M.alfredi)~"Hitchhiker Species:") +
  theme(legend.key=element_blank())

# 2nd plot
p2 <- ggplot(data = new_data %>% 
               filter(Species !="Sharksucker_Remora")) +
  geom_line(mapping = aes(x = Date, y = Proportion, group = Species, colour = Species)) + 
  scale_colour_manual(values=c(Golden_Trevally="goldenrod2", Red_Snapper="firebrick2", Juvenile_Remora="darkolivegreen3"), labels = c("Juvenile Remora", "Golden Trevally", "Red Snapper")) + 
  xlab("") + ylab("Proportion") + labs(colour = "") + theme(legend.position="top") +  theme(legend.key=element_blank()) +
  theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))

# 3rd plot
p3 <- ggplot(data = new_data_counts) +
  geom_bar(mapping = aes(x = Date, y = Count), stat =
             'identity') +
  xlab("Date (2015-2019)") +  ylab("Total"~italic
                                   (M.alfredi)~"Sightings") +
  draw_plot_label(label =c("C") + theme(axis.text.x =          
                                          element_text(angle = 90, vjust = 0.5, hjust   = 1))
                  
# The grid
grid.arrange(p1,p2,p3)

Answer 1

我建议你使用labs(..., tag = ...)和theme(plot.tag = element_text())。该代码显示了如何格式化主标题（此处以 为中心hjust = 0.5）和函数内的标签theme()。请参阅下面的可重现示例：

# Packages
library(ggplot2)
library(gridExtra)
# library(cowplot) # not necessary here

# Plots
p1 <- ggplot() +
  labs(title = "plot 1", tag = "A") +
  theme(plot.title = element_text(hjust = 0.5),
        plot.tag = element_text())

p2 <- ggplot() +
  labs(title = "plot 2", tag = "B") +
  theme(plot.title = element_text(hjust = 0.5),
        plot.tag = element_text()) 

grid.arrange(p1, p2)

如果您希望标签（A、B、C）位于绘图区域内，本文建议使用plot.tag.position = c(x, y)。参见示例：

p3 <- ggplot() +
  labs(title = "plot 3", tag = "C") +
  theme(plot.title = element_text(hjust = 0.5),
        plot.tag = element_text(),
        plot.tag.position = c(0.1, 0.8))
p3