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.

Practice

1. What is the main purpose of using geom_line() in an R ggplot?

easy

A. To connect data points with lines to show trends or changes over time

B. To create bar charts showing counts of categories

C. To add labels to points on a scatter plot

D. To fill areas under curves with color

Solution

1. Step 1: Understand the role of geom_line()

   geom_line() is used in ggplot2 to draw lines connecting data points, which helps visualize trends or changes over time.

2. Step 2: Compare with other geoms

   Other geoms like geom_bar() create bar charts, and geom_text() adds labels, so they do not connect points with lines.

3. Final Answer:

   To connect data points with lines to show trends or changes over time -> Option A

4. Quick Check:

   Line plots connect points = A [OK]

Hint: Remember: geom_line connects points in order [OK]

Common Mistakes:

• Confusing geom_line with geom_bar
• Thinking geom_line adds labels
• Using geom_line to fill areas

2. Which of the following is the correct syntax to add a line plot layer to a ggplot object named p?

easy

A. p + geom_line()

B. p + geom_point()

C. p + geom_bar()

D. p + geom_text()

Solution

1. Step 1: Identify the correct geom for line plots

   geom_line() is the function used to add line plots in ggplot2.

2. Step 2: Check the syntax for adding layers

   In ggplot2, layers are added using the + operator, so p + geom_line() correctly adds a line plot layer.

3. Final Answer:

   p + geom_line() -> Option A

4. Quick Check:

   Adding line plot layer = p + geom_line() [OK]

Hint: Add layers with + and use geom_line for lines [OK]

Common Mistakes:

• Using geom_point instead of geom_line for lines
• Forgetting the + operator
• Using geom_bar for line plots

3. What will be the output of the following R code?

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

medium

A. A scatter plot with points at (1,2), (2,5), (3,3)

B. A bar chart with bars at x=1,2,3 with heights 2,5,3

C. An error because geom_line requires grouping

D. A line plot connecting points (1,2), (2,5), and (3,3)

Solution

1. Step 1: Understand the data and mapping

   The data frame has x values 1, 2, 3 and y values 2, 5, 3. The aes maps x and y for plotting.

2. Step 2: What does geom_line() do here?

   geom_line() connects the points in order of x, so it draws a line through (1,2), (2,5), and (3,3).

3. Final Answer:

   A line plot connecting points (1,2), (2,5), and (3,3) -> Option D

4. Quick Check:

   geom_line connects points = C [OK]

Hint: geom_line connects points in order of x [OK]

Common Mistakes:

• Thinking geom_line creates scatter plot
• Expecting bar chart output
• Assuming error without grouping for simple data

4. Consider this code snippet:

library(ggplot2)
data <- data.frame(time = c(1, 2, 1, 2), value = c(3, 4, 5, 6))
ggplot(data, aes(x = time, y = value)) + geom_line()

What is the main issue with this plot?

medium

A. The plot will be empty because data is invalid

B. The plot will show two lines overlapping because grouping is missing

C. The plot will show a single line connecting all points correctly

D. The code will produce a syntax error

Solution

1. Step 1: Analyze the data structure

   The data has repeated time values (1 and 2) with different values, indicating multiple groups but no grouping variable specified.

2. Step 2: Effect of missing group aesthetic

   Without specifying a group, geom_line() connects points in the order they appear, causing lines to overlap or connect incorrectly.

3. Final Answer:

   The plot will show two lines overlapping because grouping is missing -> Option B

4. Quick Check:

   Missing group causes overlapping lines = D [OK]

Hint: Add group aesthetic to separate lines [OK]

Common Mistakes:

• Assuming syntax error instead of grouping issue
• Expecting correct single line without group
• Thinking data is invalid or empty

5. You have a dataset with daily sales for two stores over a week:

sales <- data.frame(
  day = rep(1:7, 2),
  store = rep(c('A', 'B'), each = 7),
  sales = c(10, 12, 15, 14, 13, 16, 18, 8, 9, 11, 10, 12, 13, 15)
)

How do you create a line plot showing sales trends for both stores with different colored lines?

hard

A. ggplot(sales, aes(x = day, y = sales)) + geom_line()

B. ggplot(sales, aes(x = day, y = sales, group = store)) + geom_line()

C. ggplot(sales, aes(x = day, y = sales, color = store)) + geom_line()

D. ggplot(sales, aes(x = day, y = sales, fill = store)) + geom_line()

Solution

1. Step 1: Identify the need for separate lines by store

   To show sales trends for each store, lines must be grouped by store and visually distinct.

2. Step 2: Use color aesthetic to differentiate stores

   Mapping color = store inside aes() creates separate colored lines for each store automatically grouping data.

3. Final Answer:

   ggplot(sales, aes(x = day, y = sales, color = store)) + geom_line() -> Option C

4. Quick Check:

   Color by store for separate lines = B [OK]

Hint: Use color aesthetic to separate lines by group [OK]

Common Mistakes:

• Omitting color causes single line for all data
• Using fill aesthetic instead of color for lines
• Not grouping lines by store