Legend control in R Programming - Mini Project: Build & Apply

Legend control

📖 Scenario: You are creating a simple scatter plot to compare two groups of data points. You want to add a legend to clearly show which color represents which group.

🎯 Goal: Build a scatter plot with two groups of points and add a legend that correctly labels each group.

📋 What You'll Learn

Create two numeric vectors called group1 and group2 with the exact values specified.

Create a vector called labels with the exact group names.

Plot the points for both groups with different colors.

Add a legend that uses the labels vector and matches the colors used in the plot.

💡 Why This Matters

🌍 Real World

Legends help people understand what different colors or symbols mean in charts, like showing sales by region or comparing groups in a study.

💼 Career

Data analysts and scientists often create plots with legends to communicate insights clearly to others.

1

DATA SETUP: Create data vectors

Create two numeric vectors called group1 with values c(2, 4, 6, 8) and group2 with values c(1, 3, 5, 7).

R Programming

# Create the vectors group1 and group2
# Your code here

Hint

Use the c() function to create numeric vectors with the exact values.

2

CONFIGURATION: Create labels vector

Create a character vector called labels with the exact values c("Group A", "Group B").

R Programming

group1 <- c(2, 4, 6, 8)
group2 <- c(1, 3, 5, 7)
# Create the labels vector
# Your code here

Hint

Use c() to create a character vector with the exact group names.

3

CORE LOGIC: Plot points with colors

Use plot() to plot group1 points in red with pch=16. Then add group2 points in blue using points() with pch=17.

R Programming

group1 <- c(2, 4, 6, 8)
group2 <- c(1, 3, 5, 7)
labels <- c("Group A", "Group B")
# Plot group1 points in red
# Add group2 points in blue
# Your code here

Hint

Use plot() for the first group and points() to add the second group with different colors and symbols.

4

OUTPUT: Add legend to the plot

Add a legend to the top right of the plot using legend() with labels, colors c("red", "blue"), and point characters c(16, 17).

R Programming

group1 <- c(2, 4, 6, 8)
group2 <- c(1, 3, 5, 7)
labels <- c("Group A", "Group B")
plot(group1, rep(1, length(group1)), col = "red", pch = 16, xlim = c(0, 10), ylim = c(0.5, 1.5), ylab = "", xlab = "Value", yaxt = "n")
points(group2, rep(1.2, length(group2)), col = "blue", pch = 17)
# Add a legend to the top right
# Your code here

Hint

Use legend() with the exact parameters to match colors and symbols to the labels.

Practice

1. What is the main purpose of the legend() function in R plotting?

easy

A. To explain the meaning of colors or symbols used in the plot

B. To change the plot background color

C. To add titles to the plot axes

D. To save the plot as an image file

Solution

1. Step 1: Understand the role of legend in plots

   A legend helps viewers understand what different colors or symbols represent in a plot.

2. Step 2: Identify the function purpose

   The legend() function in R adds this explanatory box to the plot.

3. Final Answer:

   To explain the meaning of colors or symbols used in the plot -> Option A

4. Quick Check:

   legend() explains plot symbols/colors [OK]

Hint: Legend explains plot symbols and colors clearly [OK]

Common Mistakes:

• Confusing legend() with title() or axis labeling functions
• Thinking legend() changes plot colors
• Assuming legend() saves plots

2. Which of the following is the correct syntax to add a legend at the top right corner with labels "A" and "B" in R?

easy

A. legend(topright, labels = c("A", "B"))

B. legend("topright", legend = c("A", "B"))

C. legend(position = "topright", c("A", "B"))

D. legend(c("A", "B"), position = "topright")

Solution

1. Step 1: Recall legend() function syntax

   The first argument is the position as a string, then legend= specifies labels.

2. Step 2: Check each option

   legend("topright", legend = c("A", "B")) uses correct syntax: legend("topright", legend = c("A", "B")). Others misuse argument names or order.

3. Final Answer:

   legend("topright", legend = c("A", "B")) -> Option B

4. Quick Check:

   Correct legend() syntax = legend("topright", legend = c("A", "B")) [OK]

Hint: First argument is position string, use legend= for labels [OK]

Common Mistakes:

• Omitting quotes around position
• Using wrong argument names like labels instead of legend
• Passing labels before position

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

plot(1:3, col = c("red", "green", "blue"), pch = 16)
legend("bottomleft", legend = c("Red", "Green", "Blue"), col = c("red", "green", "blue"), pch = 16)

medium

A. A plot with colored points but legend shows empty boxes

B. A plot with default black points and no legend

C. A plot with three points colored red, green, blue and a legend at bottom left showing these colors with filled circles

D. Syntax error due to missing parameters

Solution

1. Step 1: Understand plot() parameters

   Points 1,2,3 are plotted with colors red, green, blue and filled circle symbol (pch=16).

2. Step 2: Understand legend() parameters

   Legend placed at bottom left with matching labels, colors, and symbols (filled circles).

3. Final Answer:

   A plot with three points colored red, green, blue and a legend at bottom left showing these colors with filled circles -> Option C

4. Quick Check:

   plot colors and legend match correctly [OK]

Hint: Legend colors and symbols must match plot points [OK]

Common Mistakes:

• Assuming legend uses default black color
• Forgetting to set pch in legend
• Confusing legend position strings

4. Identify the error in this R code snippet that tries to add a legend:

plot(1:5, col = 1:5, pch = 15)
legend("top", labels = c("One", "Two", "Three", "Four", "Five"), col = 1:5, pch = 15)

medium

A. The argument 'labels' should be 'legend' in the legend() function

B. The position 'top' is invalid in legend()

C. The colors 1:5 are not valid color codes

D. pch = 15 is not a valid plotting character

Solution

1. Step 1: Check legend() argument names

   The correct argument for labels is 'legend', not 'labels'. Using 'labels' causes no legend to appear.

2. Step 2: Verify position and other parameters

   Position 'top' is valid, colors 1:5 are valid R colors, pch=15 is a valid filled square symbol.

3. Final Answer:

   The argument 'labels' should be 'legend' in the legend() function -> Option A

4. Quick Check:

   Use legend=, not labels= in legend() [OK]

Hint: Use legend= for labels inside legend() [OK]

Common Mistakes:

• Using 'labels' instead of 'legend' argument
• Thinking 'top' is invalid position
• Confusing color codes with integers

5. You want to create a plot with two groups of points: group A in red circles and group B in blue triangles. How do you add a legend that clearly shows these groups with correct symbols and colors?

hard

A. legend("topright", legend = c("Group A", "Group B"), col = c(1, 2), pch = c(16, 17))

B. legend("topright", labels = c("Group A", "Group B"), colors = c("red", "blue"), symbols = c(1, 2))

C. legend("topright", legend = c("Group A", "Group B"), color = c("red", "blue"), pch = c("circle", "triangle"))

D. legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2))

Solution

1. Step 1: Identify correct legend arguments

   Use 'legend' for labels, 'col' for colors, and 'pch' for symbols (numeric codes).

2. Step 2: Match symbols and colors to groups

   Red circles use pch=1, blue triangles use pch=2, colors as "red" and "blue" strings.

3. Step 3: Check options

   legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2)) uses correct argument names and values. legend("topright", labels = c("Group A", "Group B"), colors = c("red", "blue"), symbols = c(1, 2)) uses wrong argument names. legend("topright", legend = c("Group A", "Group B"), color = c("red", "blue"), pch = c("circle", "triangle")) uses invalid pch values. legend("topright", legend = c("Group A", "Group B"), col = c(1, 2), pch = c(16, 17)) uses numeric colors 1 and 2 which are black and red, not blue.

4. Final Answer:

   legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2)) -> Option D

5. Quick Check:

   Correct legend args and matching colors/symbols [OK]

Hint: Use legend=, col= colors, pch= numeric symbols [OK]

Common Mistakes:

• Using wrong argument names like labels or colors
• Using string names for pch instead of numbers
• Using numeric colors that don't match intended colors