Ggplot2 adding simple customizable minor tick marks revisited

Question

I'm aware that this is a long-standing problem: setting unlabelled, customizable minor tick marks in ggplot2. I find this post containing a work-around to offer 2 solutions that are somewhat customizable the best, but won't work for me personally: Quick, sleek and simple way of adding minor ticks in ggplot2?.

To my understanding you just plot all major tick marks and set the labelling of some of these, the ones that are supposed to be minor tickmarks, to NULL or "". Thus, the minor tickmarks won't have a label, but the major tickmarks will. This works, but I would like my minor tickmarks to be smaller than major tickmarks and since you cant distinguish between the two here, it doesn't work.

I don't necessarily mind typing out where the minor ticks should be placed each time, as long as this is easy to do consistently.

Given the following fictional dataset and code, is it possible to make minor y-axis ticks here inbetween the major ticks at intervals to lets say every 0.25 that are smaller than the major ticks?

library(ggplot2)

# Create some sample data
set.seed(123)
data <- data.frame(group = rep(c("A", "B"), each = 50),
                   value = c(rnorm(50, mean = 10), rnorm(50, mean = 20)))

# Create the plot using ggplot2
p <- ggplot(data, aes(x = group, y = value)) +
  geom_boxplot()

# Duplicate the y-axis
p + scale_y_continuous(sec.axis = dup_axis()) + theme_classic()

Answer 1

You could use the ggh4x package with ggh4x.axis.ticks.length.minor with rel of for example 0.5 to make smaller ticks at certain minor_breaks like this:

library(ggplot2)
library(ggh4x)
# Create some sample data
set.seed(123)
data <- data.frame(group = rep(c("A", "B"), each = 50),
                   value = c(rnorm(50, mean = 10), rnorm(50, mean = 20)))

# Create the plot using ggplot2
p <- ggplot(data, aes(x = group, y = value)) +
  geom_boxplot()

# Duplicate the y-axis
p + scale_y_continuous(
  minor_breaks = seq(8, 22, by = 0.25),
  breaks = seq(8, 22, by = 4), limits = c(8, 22),
  guide = "axis_minor"
) +
theme(ggh4x.axis.ticks.length.minor = rel(0.5)) + 
theme_classic()

^{Created on 2023-01-09 with reprex v2.0.2}

Answer 2

The {ggh4x} package has a guide function for adding minor tick marks.

For basic usage, you can add it by using the y = "axis_minor" in the guides() function.

library(ggplot2)
library(ggh4x)

# Create some sample data
set.seed(123)
data <- data.frame(group = rep(c("A", "B"), each = 50),
                   value = c(rnorm(50, mean = 10), rnorm(50, mean = 20)))

# Create the plot using ggplot2
p <- ggplot(data, aes(x = group, y = value)) +
  geom_boxplot()

p + guides(y = "axis_minor")

If you need more control over where they are placed, use the minor_breaks argument of the scale. If you want to control the length of the minor breaks, you can use the ggh4x.axis.ticks.length.minor theme setting. Note that it specifies the minor ticks length relative to the major tick length.

p + 
  scale_y_continuous(
    minor_breaks = scales::breaks_width(1),
    guide = guide_axis_minor()
  ) +
  theme(ggh4x.axis.ticks.length.minor = rel(2))

^{Created on 2023-01-09 with reprex v2.0.2}

Disclaimer: I'm the author of ggh4x. Also, the guide function might become deprecated in 2 cycles of ggplot2 development.