ggplot2: How to integrate legends for scales with parsed labels? - r

Question

Problem
In ggplot2, legends for different scales are usually integrated into a single, combined legend whenever possible. This worked fine for me so far. However, when I try parsing the scale labels to include mathematical symbols in the legend, this does not seem to work.
See this example:
# example data
d <- data.frame(x = 1:3, y = rep(0,3), f = c("a[1]", "a[2]", "a[3]"))
# plot
p <- ggplot(data = d, aes(x = x, y = y, color = f, shape = f)) +
geom_point() +
guides(
color = guide_legend(title = "F"),
shape = guide_legend(title = "F")
)
The following gives the plot with custom values for shapes/colors and with the legends combined as intended.
# plot + custom shapes/colors
p +
scale_color_manual(name = "F", values = c("red", "blue", "green")) +
scale_shape_manual(name = "F", values = c(16, 15, 18))
However, when parsing the labels, the labels come out as expected, but the legends are no longer combined.
# plot + custom shapes/colors + parsed labels
parse.labels <- function(x) parse(text = x)
p +
scale_color_manual(name = "F", labels = parse.labels, values = c("red", "blue", "green")) +
scale_shape_manual(name = "F", labels = parse.labels, values = c(16, 15, 18))
Note that the result is the same with scale_._discrete instead of scale_._manual. Similarly, specifying identical names for the two scales with guides(shape = guide_legend(title = "F"), color = guide_legend(title = "F")) does not change this behavior.
Question
How can I integrate the two legends while maintaining the parsed labels?

Answer 1

Use scales::parse_format() instead of the parse() function from base R, and you should be fine:
library(scales)
ggplot(data = d, aes(x = x, y = y, color = f, shape = f)) +
geom_point() +
scale_color_manual(name = "F",
labels = parse_format(),
values = c("red", "blue", "green")) +
scale_shape_manual(name = "F",
labels = parse_format(),
values = c(16, 15, 18))
I think this has something to do with how parse returns an expression tagged with automatically-generated srcfile / wholeSrcref attributes by default, while parse_format does not. These additional attributes prevent the two scales from being merged together, since they are not identical.
(Using function(x) parse(x = text, srcfile = NULL) in both scales will also work, same as above, but I find the function from scales to be less verbose.)

Answer 2

I would suggest this approach using the labels argument in scale_*_discrete() and saving your values for labels in a new vector:
library(ggplot2)
# example data
d <- data.frame(x = 1:3, y = rep(0,3), f = c("a[1]", "a[2]", "a[3]"))
#Labs
lab1 <- c(expression(a[1]),
expression(a[2]),
expression(a[3]))
# plot
ggplot(data = d, aes(x = x, y = y, color = f, shape = f)) +
geom_point() +
guides(
color = guide_legend(title = "F"),
shape = guide_legend(title = "F")
)+
scale_color_discrete(labels = lab1) +
scale_shape_discrete(labels = lab1)
Output: