Faceting for subplots in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of basic facet_wrap usage

What will be the output of this R code using ggplot2's facet_wrap?

R Programming

library(ggplot2)
data <- data.frame(x = 1:6, y = c(2,3,5,7,11,13), group = rep(c('A','B'), each=3))
ggplot(data, aes(x, y)) + geom_point() + facet_wrap(~group)

A Two separate plots side by side, one for group A and one for group B

B A single plot with all points combined, no separation by group

C Two plots stacked vertically, one for group A and one for group B

D Error: facet_wrap requires a numeric variable

💡 Hint

facet_wrap splits data into multiple plots based on a factor variable.

facet_wrap(~group) creates separate plots for each unique value in 'group'. Since 'group' has 'A' and 'B', two plots appear side by side.

❓ Predict Output

intermediate

Effect of facet_grid on plot layout

What is the layout of plots produced by this code?

R Programming

library(ggplot2)
data <- data.frame(x = 1:4, y = c(10,20,30,40), type = c('X','X','Y','Y'), category = c('M','N','M','N'))
ggplot(data, aes(x, y)) + geom_point() + facet_grid(type ~ category)

A A single plot with all points combined

B Error: facet_grid requires numeric variables for rows and columns

C Two plots stacked vertically for 'type', ignoring 'category'

D A 2x2 grid of plots with rows for 'type' and columns for 'category'

💡 Hint

facet_grid uses two variables to create a grid of plots.

facet_grid(type ~ category) creates a grid with rows for each 'type' and columns for each 'category', resulting in 4 plots arranged 2 by 2.

🔧 Debug

advanced

Identify the error in facet_wrap usage

This code produces an error. What is the cause?

R Programming

library(ggplot2)
data <- data.frame(x = 1:5, y = c(5,4,3,2,1), group = c('A','B','C','D','E'))
ggplot(data, aes(x, y)) + geom_point() + facet_wrap(group)

A SyntaxError: facet_wrap requires a formula, e.g., ~group

B Error: group variable must be numeric

C Error: geom_point missing aesthetic mapping

D No error, code runs fine

💡 Hint

Check how facet_wrap expects its argument.

facet_wrap expects a formula like ~group, not just the variable name.

❓ Predict Output

advanced

Number of plots created by facet_grid

How many individual plots will this code create?

R Programming

library(ggplot2)
data <- data.frame(x = 1:8, y = rnorm(8), type = rep(c('A','B'), 4), category = rep(c('X','Y'), each=4))
ggplot(data, aes(x, y)) + geom_point() + facet_grid(type ~ category)

A 2

B 4

C 8

D 1

💡 Hint

Count unique values in 'type' and 'category'.

There are 2 unique 'type' values and 2 unique 'category' values, so 2x2=4 plots are created.

🧠 Conceptual

expert

Understanding free scales in faceting

What is the effect of setting scales = 'free' in facet_wrap or facet_grid?

A Only the x axis scales vary, y axis scales remain fixed

B All subplots share the same x and y axis scales

C Each subplot can have its own x and y axis scales independently

D The plot will show only one subplot ignoring others

💡 Hint

Think about axis scaling flexibility across subplots.

Setting scales = 'free' allows each subplot to adjust its axis scales independently, useful when data ranges differ.