How to use hjust to move the label on only one plot?-CodePudding

I am creating a multipanel graph using plot_grid of the relationship of discharge and surface area with species richness in Amazon Rivers. Here is my data:

riverssub<-structure(list(richness = c(127L, 110L, 89L, 74L, 62L, 18L, 22L, 
71L, 38L, 91L, 56L, 39L, 90L, 37L, 147L, 53L, 92L, 207L, 52L, 
126L, 79L, 32L, 100L, 181L, 83L, 690L), surface = c(33490, 4410, 
770, 164.7, 288.5, 9.85, 33.1, 750, 46.9, 970, 85.2, 39.2, 780, 
97.3, 3983.71, 220, 500, 11250, 115, 1350, 278, 23.05, 310, 2050, 
560, 34570), disch = c(2640L, 687L, 170L, 353L, 384L, 16L, 31L, 
513L, 32L, 392L, 50L, 32L, 206L, 81L, 1260L, 104L, 220L, 6100L, 
308L, 2060L, 443L, 102L, 348L, 4758L, 913L, 40487L)), class = "data.frame", row.names = c(NA, 
-26L))

Here is the code for my graphs and multiplot:

library(cowplot)
a <- ggplot(data = riverssub, aes(x = surface , y = richness))   
  geom_point()   
  scale_y_log10()  
  scale_x_log10()   
  labs(x='Surface Area (100 km\u00b2)', y="Fish Species Richness")  
  theme_bw()
b <- ggplot(data = riverssub, aes(x = disch , y = richness))   
  geom_point()   
  scale_y_log10()  
  scale_x_log10()   
  labs(x=bquote('Mean Annual Discharge'~(m^3 * s^-1)), y=" ")  
  theme_bw()
plot_grid(a, b   theme(axis.text.y = element_blank()),
          nrow = 1, align = 'h', labels="AUTO", label_y=0.97, label_x=0.1)

I want the "A" label to be in the same position on the first plot as the "B" label is on the second plot. I know I can use hjust() within plot_grid() to achieve this, although I am unsure how to do it. Can anyone help? Thanks in advance.

CodePudding user response：

Instead of fiddling around with hjust to place the labels I would suggest to add the labels on the plots before aligning them via plot_grid as already suggested by @Guillaume in his comment. One option to do so and to ensure that the labels will be put on the same relative positions would be to make use annotation_custom:

library(cowplot)
library(ggplot2)
library(magrittr)

a <- ggplot(data = riverssub, aes(x = surface, y = richness))  
  geom_point()  
  scale_y_log10()  
  scale_x_log10()  
  labs(x = "Surface Area (100 km\u00b2)", y = "Fish Species Richness")  
  theme_bw()

b <- ggplot(data = riverssub, aes(x = disch, y = richness))  
  geom_point()  
  scale_y_log10()  
  scale_x_log10()  
  labs(x = bquote("Mean Annual Discharge" ~ (m^3 * s^-1)), y = " ")  
  theme_bw()  
  theme(axis.text.y = element_blank())

list(A = a, B = b) %>%
  purrr::imap(function(x, y) x   annotation_custom(grid::textGrob(label = y, x = .05, y = .97, gp = grid::gpar(fontface = "bold")))) %>%
  plot_grid(plotlist = ., nrow = 1, align = "h")