How to add lines to connect certain data points across multiple boxplots in R?

Question

I am trying to create a graph similar to this, where one specific data point for each boxplot is connected to the next through a red line.

My current code is:

p <- ggplot(melt_opt_base, aes(factor(variable), value))
p + geom_boxplot() + labs(x = "Variable", y = "Value")

And the current graphs looks like this. Assuming the data points to connect are:

points = c(0, 0.1, 0.2, 0.3, 0, 0.2, 0.2, 0.1, 0.3)

Does anyone know how I could add a line connecting these points across the nine adjacent boxplots, so that it would look like this instead?

Answer 1

This could be achieved like so:

1. Make a dataframe with your categories and the point values
2. Pass this dataframe as data to geom_line and or geom_point

Making use of ggplot2::diamonds as example data:

library(ggplot2)

points <- aggregate(price ~ cut, FUN = mean, data = diamonds)

points
#>         cut    price
#> 1      Fair 4358.758
#> 2      Good 3928.864
#> 3 Very Good 3981.760
#> 4   Premium 4584.258
#> 5     Ideal 3457.542

ggplot(diamonds, aes(cut, price)) + 
  geom_boxplot() +
  geom_point(data = points, color = "red") +
  geom_line(data = points, aes(group = 1), color = "red")