signs
In signs: Insert Proper Minus Signs

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

library(scales)
library(signs)
library(dplyr)
library(ggplot2)
library(ggrepel)

The Basics

Using signs is simple, especially if you're familiar with functions like number(), number_format(), comma(), comma_format(), percent(), and percent_format() from the scales package. It simply provides two new functions to complement these: signs() and signs_format().

Points in group 1 are labeled with a true Unicode minus glyph.
Points in group 2 are labeled with the traditional ASCII hyphen-minus.
Basic usage is identical.

theme_set(theme_gray())
theme_update(
  panel.grid.minor = element_blank(),
  axis.text.y      = element_blank(),
  axis.ticks.y     = element_blank()
)

p <- 
  ggplot(sleep) +
  aes(group, extra) +
  geom_point() +
  xlab("Drug") +
  ylab("Extra Sleep (hours)")

label_hours <- function(mapping) {
  geom_text_repel(
    mapping,
    nudge_x       = -.1,
    direction     = "y",
    segment.size  = .4,
    segment.color = "grey75",
    hjust         = "right"
  )
}

p +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra, accuracy = .1),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )

Other Number Formats

You can use any formatting function with signs::signs()``r "\u2014"not just scales::number(). Let's assume everyone gets 8 hours of sleep, so we can label the points as percentages.

p +
  ylab("Extra Sleep (% over 8 hours)") +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra / 8, accuracy = .1, format = scales::percent),
        group == 2 ~ percent(extra / 8, accuracy = .1)
      )
    )
  )

Or we can muliply by days in a year and use scales::comma().

p +
  ylab("Extra Sleep (hours / year)") +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra * 365, format = scales::comma),
        group == 2 ~ comma(extra * 365)
      )
    )
  )

format can be any function that takes a numeric vector and returns a character vector.

Matching by Position

Note that format and all other options (see below) come after the dots; this way you can enjoy the same matching by position you know from scales. For example, if you prefer the simplicity of number(x, 1), you can simply use signs(x, 1).

x <- seq(-4, 4)

number(x, 1) # first argument is accuracy
signs(x, 1)  # first argument is accuracy

Other Arguments

signs::signs() offers 3 other arguments for convenience:

add_plusses
trim_leading_zeros
label_at_zero

(#3 is addressed below under Axis Labels.)

Sometimes, as with this dataset, you want to show change from a baseline. You might not only want to include a r "\u2212" in front of negative numbers, but a + in front of positive numbers as well. This is as simple as add_plusses = TRUE.

p +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra, accuracy = .1, add_plusses = TRUE),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )

If all values are on the interval (r "\u2212"1, 1), it can be more compact to remove leading zeros. Do this with trim_leading_zeros = TRUE:

p +
  ylim(-.8, .8) +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra, accuracy = .1, trim_leading_zeros = TRUE),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  ) +
  theme(
    axis.text.y = element_blank(),
    axis.ticks.y = element_blank()
  )

Axis Labels

You can also use Unicode minus signs on an entire axis. This function is called signs::signs_format(), by analogy to scales::number_format(), scales::percent_format(), and the rest of the _format() functions. Note that it accepts the same optional arguments as signs::signs() as well.

theme_update(
  axis.text.y = element_text(hjust = 1)
)

p +
  scale_y_continuous(
    limits = c(-.8, .8),
    breaks = seq(-.8, .8, by = .2),
    labels = signs_format(
      accuracy           = .1,
      add_plusses        = TRUE,
      trim_leading_zeros = TRUE
    )
  ) +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(
          extra,
          accuracy           = .1,
          add_plusses        = TRUE,
          trim_leading_zeros = TRUE
        ),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )

You may want to treat zero itself differently, particularly when every other value has either a plus or a minus. Maybe you'll be extra pedantic about it with label_at_zero = "symbol" (notice the y-axis labels below, not the data point labels):

p +
  scale_y_continuous(
    limits = c(-4, 6),
    breaks = seq(-4, 6, by = 1),
    labels = signs_format(
      add_plusses = TRUE,
      label_at_zero = "symbol"
    )
  ) +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(
          extra,
          accuracy = .1,
          add_plusses = TRUE
        ),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )

Or, especially if the location of zero is already obvious, you might want to leave it blank with label_at_zero = "blank":

p +
  scale_y_continuous(
    limits = c(-4, 6),
    breaks = seq(-4, 6, by = 1),
    labels = signs_format(
      add_plusses = TRUE,
      label_at_zero = "blank"
    )
  ) +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(
          extra,
          accuracy = .1,
          add_plusses = TRUE
        ),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )

Setting Options Globally

You can set format, add_plusses, trim_leading_zeros, or label_at_zero globally for a script with options():

options(
  signs.format             = scales::number,
  signs.add.plusses        = TRUE,
  signs.trim.leading.zeros = TRUE,
  signs.label.at.zero      = "none"
)

p +
  scale_y_continuous(
    limits = c(-.8, .8),
    breaks = seq(-.8, .8, by = .2),
    labels = signs_format(accuracy = .1, label_at_zero = "blank")
  ) +
  label_hours(
    aes(
      label = case_when(
        group == 1 ~ signs(extra, accuracy = .1),
        group == 2 ~ number(extra, accuracy = .1)
      )
    )
  )