insert legend color square into r ggplot axis label

Question

I've found lots of great documentation for how to insert special characters and Greek letters into axis labels in ggplot in R, but nothing for pasting legend colors into the axis labels. I am creating a graph with a slightly complicated x-axis, and a collaborator suggested formatting the axis with a colored square (identical to the figure legend) in the axis label text so readers can reference which axis label refers to which data series.

Here's what the figure legend currently looks like:

[ORANGE SQUARE] Series 1

[BLUE SQUARE] Series 2

And here's what I'd like the x-axis label to look like:

Extent in km [ORANGE SQUARE] or km/yr [BLUE SQUARE]

Is it possible to do this kind of graphical manipulation in R, or do I need to create this kind of label in some other image processing software? Here's the plotting code I'm using, with some fake data:

SERIES1 <- as.data.frame(sample(1:100, 100, replace=TRUE)) %>% mutate(source="SERIES1") 
SERIES2 <- as.data.frame(sample(1:1000, 100, replace=TRUE)) %>% mutate(source="SERIES2")
SERIES3 <- as.data.frame(sample(1:10000, 100, replace=TRUE)) %>% mutate(source="SERIES3")
colnames(SERIES1) <- c("value","source")
colnames(SERIES2) <- c("value","source")
colnames(SERIES3) <- c("value","source")
gg_df <- rbind(SERIES1, SERIES2, SERIES3)

fig1A <- ggplot(gg_df) + 
  geom_density(alpha=0.5, size=0.2, aes(x=value, y=..scaled.., fill=factor(source, labels=c('SERIES1','SERIES2','SERIES3')))) + 
  scale_x_continuous(limits=c(0,16000), breaks=c(seq(0, 16000, by=2000))) + 
  labs(x='Extent (km) or (km/dec)',y='Density') +
  theme_bw() +
  theme(legend.position=c(0.8, 0.85), legend.title=element_blank()) + 
  scale_fill_brewer(type='qual',palette='Dark2')

Jan Boyer Jan Boyer · Accepted Answer · 2017-11-21T17:35:19

The cowplot package has nice functions for annotating on top of ggplots

require(cowplot)

first, add spaces to your x label to make room for squares

fig1A <- ggplot(gg_df) + 
  geom_density(alpha=0.5, size=0.2, aes(x=value, y=..scaled..,
        fill=factor(source, labels=c('SERIES1','SERIES2','SERIES3')))) + 
  scale_x_continuous(limits=c(0,16000), breaks=c(seq(0, 16000, by=2000))) + 
  labs(x='Extent (km)     or (km/dec)     ',y='Density') + 
  theme_bw() +
  theme(legend.position=c(0.8, 0.85), legend.title=element_blank()) + 
  scale_fill_brewer(type='qual',palette='Dark2')

Make a dataframe containing x and y locations for your squares

squares <- data.frame(x = c(0.53, 0.75),  y = c(0.017,0.017))

use the ggdraw function in cowplot to draw your graph and then annotate squares on top of it. Unlike annotate and geom_rect in ggplot, which allow annotation only inside the plot area, you can annotate anywhere on your figure using cowplot. The location of your squares is given using a 0 - 1 scale that goes from left to right and bottom to top on your plot - you will probably have to adjust the numbers depending on what size you are saving your graph as.

ggdraw(fig1A) + 
geom_rect(data = squares, aes(xmin = x, xmax = x + .02, 
                          ymin = y, ymax = y + .02),
        fill = c("orange", "blue"))

insert legend color square into r ggplot axis label

1 Answers