How to keep default axis labels but add an additional label in ggplot2

Question

I would like to keep the default labels ggplot2 provides for Y-axis below, but always have a Y-axis tick and/or label at y = 100 to highlight the horizontal line intercept.

library(ggplot2) maxValue <- 1000 df <- data.frame( var1 = seq(1, maxValue, by = 25), var2 = seq(1, maxValue, by = 50) ) ggplot(df, aes(x = var1, y = var2)) + geom_point() + geom_hline(yintercept = 100, color = "red")

^{Created on 2022-04-09 by the reprex package (v2.0.1.9000)}

Expected output:

Note that maxValue can be anything. So the solution to just increase in steps of 100 doesn't work. For example:

plot <- plot + scale_y_continuous( breaks = seq(0, max(df$y) + 100, 100), labels = as.character(seq(0, max(df$y) + 100, 100)) )

This is because if the max value is 10000 or a similar big number like that, the number of labels will be overwhelming. This is why I would like to stay with the default Y-axis labels that ggplot2 provides and only add a single additional label at y = 100.

If it's OK to have the label on the opposite (= right) side of your y-axis, you can use dup_axis like so: ggplot() + ... + scale_y_continuous(sec.axis = dup_axis(breaks = maxValue)) — user18309711
– user18309711, Commented Apr 9, 2022 at 18:46

AdroMine · Accepted Answer · 2022-04-09 18:59:06Z

By default ggplot2 will compute the default axis breaks in the following manner (Refer to this answer for more details):

labeling::extended(min(df$var1), max(df$var1), m = 5))

We can just add your custom value 100 to this vector and pass it to scale_y_continous

def_breaks <- labeling::extended(min(df$var1), max(df$var1), m = 5) ggplot(df, aes(x = var1, y = var2)) + geom_point() + geom_hline(yintercept = 100, color = "red") + scale_y_continuous(breaks = c(100, def_breaks), # pass to minor breaks so that they are not messed up minor_breaks = def_breaks)

good answer. a possible improvement would be to add in logic to avoid overlapping custom and default values (e.g. the custom value must be at least 5% of the axis range different from each default value to display the custom value and all of the default values; otherwise don't display one of the 2 similar values).

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