How to create a boxplot for multiple dependent variables from the WeightLoss dataset?

Question

I'm trying to make a boxplot from the WeightLoss dataset from the car package. The variables of interest are separated by month, so I made boxplots by month separately, with all the groups (Control, Diet, Diet and Exercise) showing. I only want to get the boxplot of the group Diet and their weight loss across the span of 3 months in 1 boxplot instead of 3. To clarify, I need ylab="Weight loss" and xlab="Month" as my axes. I have no idea on how to do this though.

This is the first feasible set of boxplots I was able to make, but they're separated by month and all the groups show up in the boxplot.

library(car)
library(DescTools)

boxplot(wl1 ~ group, data=WeightLoss, 
        main="Weight Loss after 1 month",
        ylab="Weight loss",
        xlab="Group")
boxplot(wl2 ~ group, data=WeightLoss, 
        main="Weight Loss after 2 months",
        ylab="Weight loss",
        xlab="Group")
boxplot(wl3 ~ group, data=WeightLoss, 
        main="Weight Loss after 3 months",
        ylab="Weight loss",
        xlab="Group")

I tried to separate the group I only wanted like this:

boxplot(wl1 ~ group$Diet, data=WeightLoss,  
        main="Weight Loss after 1 month",
        ylab="Weight loss",
        xlab="Group")

but I keep on getting this error:

Error in group$diet : $ operator is 
    invalid for atomic vectors

Answer 1

We could first subset to diet group, then reshape the months into long format (cbinding an ID column first).

library(car)  ## to load `WeightLoss` data

subset(cbind(WeightLoss, id=seq_len(nrow(WeightLoss))), group == 'Diet') |>
  reshape(varying=c("wl1", "wl2", "wl3"), timevar='months', dir='long', sep='') |>
  boxplot(wl ~ months, data=_)

The plot shows the weight loss in the respective month.

Answer 2

Try

boxplot(wl1 ~ as.character(group),
         data= 
   WeightLoss[WeightLoss$group=="Diet", ],
         main="Weight Loss after 1 month",
         ylab="Weight loss",
         xlab="Group")