R : Plot a boxplot with a continuous x axis : choose an interval for each box

Question

I am currently working on R, and I have some troubles with the boxplot from the package ggplot2.

What I want to do is to plot the NO2 concentration depending on the speed of the vehicles on the road. So I have a continuous x-axis and a continuous y-axis. When I use geom_boxplot, I get those graphs

ggplot(df, aes(x=Speed, y=Concentration)) + 
  geom_boxplot() +
  scale_x_continuous(limits = c(0, 100)) +
  scale_y_continuous(limits = c(0,500))

We can see that the boxes are randomly disposed on this graph What I want is to get a different boxplot every 20 km/h between 0 and 100 km/h.

I have tried different things seen on other topics from the forum, like :

aes(group = cut_width(Speed, 20))

but nothing is changing and my boxes won't be positioned every 20 km/h.

I am not sure that my explanations are very clear, please do not hesitate to ask if you don't understand something.

It's been a few days that I'm trying to solve this problem, and I would be very grateful if someone could help me on that issue.

Thank you, Valentine

Edit : Here is a code to create a dataset, and a picture of the result.

df = data.frame(matrix(ncol = 2, nrow = 20))
colnames(df) = c("Speed", "Concentrations")
df$Speed = runif(20, 0,100)
df$Concentrations = runif(20,0,500)
ggplot(df, aes(x = Speed, y = Concentrations)) + geom_boxplot(aes(group = cut_width(Speed, 20)))

The result is here. What I want is to have a box at Speed 20, 40, 60, 80.

Please post a representative sample of your data that we can use to help. The boxes are grouped by the variable you assign to x, so in this case it's Speed. So that's where you would need to apply cut_width. — camille
Sure, here is a link to the dataframe : drive.google.com/drive/folders/… I made a mistake while posting, I just edited this Speed mistake, thanks. — Valentine
Please post a self-contained example that doesn't require us to download data, and ideally use the reprex package. See also here: reprex.tidyverse.org/articles/reprex-dos-and-donts.html — Claus Wilke
I added a code to create the dataset on my post, since I don't understand how to use reprex :(. I hope it is fine tho. Thanks. — Valentine

Josh Kay Josh Kay · Accepted Answer · 2018-08-08T21:26:34

Consider adding the following discrete variable to your data instead of applying cut_width() in your ggplot commands:

df$Speed_Cat = cut_width(df$Speed, 20)

Then your plot will be constructed via:

ggplot(df, aes(x = Speed_Cat, y = Concentrations)) + 
geom_boxplot() + 
scale_x_discrete(labels=seq(0,100,20))

Just know what you want your cuts to represent! Buckets become [-10,10], (10,30], ..., but you can always adjust these when you create the variable in your data.

R : Plot a boxplot with a continuous x axis : choose an interval for each box

1 Answers