Boxplot by group and then column in r

Question

How do I make a boxplot such that each group of boxes in the boxplot contains columns of variables from a dataframe.

For example using the mpg dataset:

head(mpg)

# A tibble: 234 x 11
   manufacturer model      displ  year   cyl trans      drv     cty   hwy fl    class  
   <chr>        <chr>      <dbl> <int> <int> <chr>      <chr> <int> <int> <chr> <chr>  
 1 audi         a4           1.8  1999     4 auto(l5)   f        18    29 p     compact
 2 audi         a4           1.8  1999     4 manual(m5) f        21    29 p     compact
 3 audi         a4           2    2008     4 manual(m6) f        20    31 p     compact
 4 audi         a4           2    2008     4 auto(av)   f        21    30 p     compact
 5 audi         a4           2.8  1999     6 auto(l5)   f        16    26 p     compact
 6 audi         a4           2.8  1999     6 manual(m5) f        18    26 p     compact
 7 audi         a4           3.1  2008     6 auto(av)   f        18    27 p     compact
 8 audi         a4 quattro   1.8  1999     4 manual(m5) 4        18    26 p     compact
 9 audi         a4 quattro   1.8  1999     4 auto(l5)   4        16    25 p     compact
10 audi         a4 quattro   2    2008     4 manual(m6) 4        20    28 p     compact
# ... with 224 more rows

So within each cyl group (4,5,6,8), I want to have boxplots for each variable/column cty,hwy, and displ.

Usually, one will set the fill in ggplot to be a factor variable but in this case, I have 3 variables.

It should look something like this:

Answer 1

You need to tranform your data to long format on your three variables. Here an example with data.table and melt function, but you will easily find the same with tydr:

library(ggplot2)
library(data.table)

mpg <- setDT(copy(mpg))
mpg_plot <- melt(mpg,measure.vars = c("cty","hwy","displ"),value.name = "val",variable.name = "var")

ggplot(mpg_plot, aes(x = as.factor(cyl),y = val,fill = var))+
  geom_boxplot()+
  theme_light()