Line plots (geom_line) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this ggplot2 line plot code?

Consider the following R code using ggplot2. What will the plot display?

R Programming

library(ggplot2)
data <- data.frame(x = 1:5, y = c(3, 5, 2, 8, 7))
ggplot(data, aes(x = x, y = y)) + geom_line()

A A line plot connecting points (1,3), (2,5), (3,2), (4,8), (5,7) in order

B A scatter plot with points only, no lines

C A bar plot with bars at x positions 1 to 5 with heights 3,5,2,8,7

D An error because geom_line requires grouping

💡 Hint

Think about what geom_line() does with x and y aesthetics.

geom_line() connects points in the order of the x variable, creating a line plot.

🧠 Conceptual

intermediate

Why might you need to use the group aesthetic in geom_line()?

In ggplot2, when plotting multiple lines with geom_line(), why is the group aesthetic important?

A To change the color of the lines automatically

B To tell ggplot which points belong to the same line so lines don't connect across groups

C To add labels to the lines

D To sort the data points before plotting

💡 Hint

Think about what happens if ggplot doesn't know which points belong together.

The group aesthetic tells ggplot which points form a single line. Without it, lines may connect points from different groups incorrectly.

🔧 Debug

advanced

What error does this code produce?

Examine this R code snippet:

library(ggplot2)
data <- data.frame(time = 1:4, value = c(2, 4, 6, 8))
ggplot(data, aes(x = time, y = value)) + geom_line(color = 5)

What error or warning will this code produce?

A Warning: ignoring unknown parameter 'color'

B No error; line color will be set to color number 5

C Error: 'color' must be a character string or a valid color name

D Error: object 'color' not found

💡 Hint

Check what type of value color expects in geom_line().

The color argument expects a color name or hex code as a string, not a number. Passing 5 causes an error.

📝 Syntax

advanced

Which option produces a valid line plot with two groups?

Given this data frame:

df <- data.frame(
  time = rep(1:3, 2),
  value = c(2, 3, 5, 1, 4, 6),
  group = rep(c('A', 'B'), each = 3)
)

Which ggplot2 code correctly plots lines for each group?

A ggplot(df, aes(x = time, y = value, group = group)) + geom_line()

B ggplot(df, aes(x = time, y = value)) + geom_line()

C ggplot(df, aes(x = time, y = value, group = 1)) + geom_line()

D ggplot(df, aes(x = time, y = value, color = group)) + geom_line()

💡 Hint

Think about how to show separate lines by group with color.

Option D uses color = group which also sets grouping automatically, producing separate colored lines for each group.

🚀 Application

expert

How many lines will this plot display?

Given this data frame and plot code:

df <- data.frame(
  day = rep(1:4, 3),
  score = c(5, 6, 7, 8, 3, 4, 5, 6, 9, 10, 11, 12),
  player = rep(c('X', 'Y', 'Z'), each = 4)
)
ggplot(df, aes(x = day, y = score, group = player)) + geom_line()

How many separate lines will appear in the plot?

A 3

B 1

C 4

D 12

💡 Hint

Count the unique groups defined by player.

There are 3 unique players, so 3 separate lines will be drawn.