Manually position labels in ggplot legend

Question

I would like to know if it's possible to manually position the labels in a legend in ggplot?

My example is this: I have a data on countries, and I'm doing a 100% stacked bar for each continent so I have:

dt <- data.table(continent = c(rep('Africa', 2), rep('Asia', 3), rep('Europe', 4)),
                 country = c('Nigeria', 'Kenya',
                             'China', 'India', 'Japan',
                             'Germany', 'Sweden', 'Spain', 'Croatia'),
                 value = runif(9, 0, 10),
                 number=(1:9))


ggplot(data=dt, 
       aes(x = continent, y = value, fill = as.factor(number))) +
  geom_bar(stat = "identity", position = "fill", color='white', width=0.3 ) + 
  labs(x = '', y = 'Percentage') +
  scale_y_continuous(expand=c(0,0)) +
  scale_fill_manual('Country',
                    labels = dt[, country],
                    values = (grDevices::colorRampPalette(c('#BB16A3', '#f8e7f5')))(9)) +
  theme(legend.position='bottom', aspect.ratio = 1) +
  guides(fill = guide_legend(title.position="top", title.hjust = 0.5, reverse=T)) +
  coord_flip()

So my question is, is it possible to re-position the labels in the legend so that countries of each continent are in a separate column? Or a separate row?

Thanks!

Answer 1

I notice you have a different number of countries for each continent. ggplot() can fill a legend matrix by row or by column, but I've never seen a jagged matrix with different number of cells in each row / column.

It's possible to hack something that looks like a jagged legend matrix, though. Here are some implementations. You may wish to tweak the parameters if you want to sort the continent / country labels in a specific order, or vary the spacing between legend keys, etc.

Prep work:

# define fill mapping so that it can be re-used for both top plot & legend
scale_fill_country <- 
  scale_fill_manual(labels = dt[, country],
                    values = (grDevices::colorRampPalette(c('#BB16A3', '#f8e7f5')))(9))

# create top plot (without any legend)
gg.plot <- ggplot(data = dt, 
                  aes(x = continent, y = value, fill = as.factor(number))) +
  #note: geom_col is equivalent to geom_bar(stat = "identity")
  geom_col(position = "fill", color='white', width=0.3 ) + 
  labs(x = '', y = 'Percentage') +
  scale_y_continuous(expand=c(0,0)) +
  scale_fill_country +
  theme_classic() +
  theme(legend.position = "none") +
  coord_flip()

Modify data source for legend:

library(dplyr)
dt.legend <- dt %>% 

  # pad with empty rows so that there are equal number of countries under
  # each continent
  group_by(continent) %>% 
  arrange(country) %>% 
  mutate(country.id = seq(1, n())) %>% 
  ungroup() %>% 
  tidyr::complete(continent, country.id, fill = list(country = " ")) %>%

  # make each empty row distinct (within the same continent), & sort them
  # after the original rows
  rowwise() %>%
  mutate(country = ifelse(country == " ", 
                          paste0(rep.int(" ", country.id), collapse = ""),
                          country)) %>%
  ungroup() %>%
  mutate(country = forcats::fct_reorder(country, country.id))

> dt.legend
# A tibble: 12 x 5
   continent country.id country   value number
   <chr>          <int> <fct>     <dbl>  <int>
 1 Africa             1 Kenya    2.02        2
 2 Africa             2 Nigeria  7.17        1
 3 Africa             3 "   "   NA          NA
 4 Africa             4 "    "  NA          NA
 5 Asia               1 China    3.21        3
 6 Asia               2 India    5.59        4
 7 Asia               3 Japan    9.31        5
 8 Asia               4 "    "  NA          NA
 9 Europe             1 Croatia  0.0131      9
10 Europe             2 Germany  0.0775      6
11 Europe             3 Spain    3.98        8
12 Europe             4 Sweden   0.703       7

Version 1: each continent in one row, labels below legend key (add axis.text.y = element_blank() to theme() if you don't want the continent label associated with each row to be shown)

gg.legend.rows1 <- ggplot(data = dt.legend,
                         aes(x = country, y = continent,
                             fill = as.factor(number))) +
  geom_tile(color = "white", size = 2) +
  facet_wrap(~ continent, scales = "free", ncol = 1) +
  scale_y_discrete(expand = c(0, 0)) +
  scale_fill_country +
  theme_minimal() +
  theme(axis.title = element_blank(),
        strip.text = element_blank(),
        panel.grid = element_blank(),
        legend.position = "none")

cowplot::plot_grid(gg.plot, gg.legend.rows1,
                   ncol = 1,
                   rel_heights = c(1, 0.3))

Version 2: each continent in one row, labels to the right of legend key (I couldn't think of a way to get the continent labels in as well for this approach, but I don't think that was required in the question anyway...)

gg.legend.rows2 <- ggplot(data = dt.legend,
       aes(x = "", y = country, fill = as.factor(number))) +
  geom_tile() +
  scale_y_discrete(position = "right", expand = c(0, 0)) +
  facet_wrap(~ interaction(continent, country, lex.order = TRUE), 
             scales = "free") +
  scale_fill_country +
  theme_minimal() +
  theme(axis.title = element_blank(),
        axis.text.x = element_blank(),
        strip.text = element_blank(),
        panel.grid = element_blank(),
        panel.spacing = unit(0, "pt"),
        legend.position = "none")

cowplot::plot_grid(gg.plot, gg.legend.rows2,
                   axis = "l", align = "v",
                   ncol = 1,
                   rel_heights = c(1, 0.2))

Version 3: each continent in one column, labels to the right of legend key (add axis.text.x = element_blank() to theme() if you don't want the continent label associated with each column to be shown)

gg.legend.columns <- ggplot(data = dt.legend,
                            aes(x = continent, y = forcats::fct_rev(country), 
                                fill = as.factor(number))) +
  geom_tile(color = "white", size = 2) +
  facet_wrap(~ continent, scales = "free", nrow = 1) +
  scale_x_discrete(position = "top", expand = c(0, 0)) +
  scale_y_discrete(position = "right", expand = c(0, 0)) +
  scale_fill_country +
  theme_minimal() +
  theme(axis.title = element_blank(),
        strip.text = element_blank(),
        panel.grid = element_blank(),
        legend.position = "none")

cowplot::plot_grid(gg.plot, gg.legend.columns, 
                   axis = "l", align = "v",
                   ncol = 1, 
                   rel_heights = c(1, 0.3))