How to insert a custom tick mark and a corresponding label on R plot?

When creating a plot, R assigns default tick marks for the axes. I wish to insert custom tick marks and labels along the default tick marks. I'd also like to further customise the inserted tick marks and labels (e.g., change size and color) so that they would differ from the default generated ones.

Here's an illustration:

library(ggplot2)

x <- c(1,2,3,4,5,6,7)
y <- c(1,2,3,4,5,6,7)
ggplot() + geom_line(aes(x,y)) + geom_point(aes(x=3.27, y=3.33), size=7, col="turquoise4")

enter image description here

In the picture, the coloured point is the most important piece of data on the graph. As such, the goal is to help the reader easily understand that data point. Ideally, the hand-drawn red tick marks would be thicker than the default ones, have the same colour as the data point, and be accompanied with the corresponding labels (i.e., 3.27 for x axis, and 3.33 for y) - again, same colour as the data point, and bigger in size, comparing to the default ones.

After having checked several resources on axis customisation, it seems that they focus on adjusting all tick marks at once - colour, orientation, size, intervals between tick marks, etc. How to create unique tick marks and labels that differ from the rest remains a mystery for me. Any help is highly appreciated

You can always set your desired breaks via the breaks argument of the scale, e.g. for your example you could add your custom break and tick to the x axis like so:

library(ggplot2)

x <- c(1, 2, 3, 4, 5, 6, 7)
y <- c(1, 2, 3, 4, 5, 6, 7)

base <- ggplot() +
  geom_line(aes(x, y)) +
  geom_point(aes(x = 3.27, y = 3.33), size = 7, col = "turquoise4")

base +
  scale_x_continuous(breaks = c(2, 4, 6, 3.27))

<!-- -->

For a more general or automatic approach you could add the custom break to the default vector of breaks which are computed via scales::breaks_extended(), i.e you could do:

base +
  scale_x_continuous(breaks = \(x) c(scales::breaks_extended()(x), 3.27))

<!-- -->

Similar you could use the labels argument to assign a vector of labels or use a formatter function to add labels to the breaks. For each case one approach to set a color and/or font size for your custom tick only or for a subset of ticks would be to use the ggtext package which allows for styling via Markdown, HTML and/or CSS ,e.g. for the manual approach you could do:

library(ggtext)

base +
  scale_x_continuous(
    breaks = c(2, 4, 6, 3.27),
    labels = c(2, 4, 6, "<span style='color: turquoise4; font-size: 20pt'>3.27</span>")
  ) +
  theme(axis.text.x = ggtext::element_markdown())

<!-- -->

Finally, for a more general or automatic approach I personally prefer to use some custom functions. Whether such an approach is worthwhile depends however on how often you have or want to use this functionality. Such an approach may look like so:

library(ggplot2)
library(ggtext)

label_highlight <- function(x, no_high) {
  x[!x %in% no_high] <- glue::glue(
    "<span style='color: turquoise4; font-size: 20pt'>{x[!x %in% no_high]}</span>"
  )
  x
}

default_breaks <- function(x, expand = .05) {
  scales::breaks_extended()(range(x) + expand * c(-1, 1) * diff(range(x)))  
}

default_x <- default_breaks(x)
default_y <- default_breaks(y)

ggplot() +
  geom_line(aes(x, y)) +
  geom_point(aes(x = 3.27, y = 3.33), size = 7, col = "turquoise4") +
  scale_x_continuous(
    breaks = c(default_x, 3.27),
    label = ~ label_highlight(.x, default_x)
  ) +
  scale_y_continuous(
    breaks =  c(default_y, 3.33),
    label = ~ label_highlight(.x, default_y)
  ) +
  theme(
    axis.text.x = ggtext::element_markdown(),
    axis.text.y = ggtext::element_markdown()
  )

<!-- -->