Legend control in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a legend in an R plot?

A legend explains the symbols, colors, or line types used in a plot so the viewer can understand what each element represents.

beginner

How do you add a basic legend to a plot in R?

Use the legend() function, specifying the position, labels, and symbols or colors to match the plot elements.

beginner

What argument in legend() controls the position of the legend?

The x argument controls the position. It can be a keyword like "topright", "bottomleft", or coordinates like c(1,2).

intermediate

How can you change the colors of legend symbols in R?

Use the col argument inside legend() to specify colors matching the plot symbols or lines.

intermediate

What does the pch argument do in the legend() function?

It sets the plotting character (symbol) used in the legend to match the points or symbols shown in the plot.

Which function is used to add a legend to a plot in R?

A plot()

B title()

C legend()

D points()

The legend() function adds a legend to an existing plot.

How do you position a legend at the top right corner of a plot?

A x = "topright"

B x = "bottomleft"

C x = c(0,0)

D x = "center"

Using x = "topright" places the legend at the top right corner.

Which argument controls the colors of symbols in the legend?

A cex

B pch

C lty

D col

The col argument sets the colors of the legend symbols or lines.

If you want the legend symbols to match the plot points, which argument do you set?

A bg

B pch

C lwd

D col

The pch argument sets the plotting character (symbol) in the legend.

Can you specify exact coordinates for the legend position in R?

A Yes, by giving numeric coordinates to the x argument

B No, only keywords like 'topright' are allowed

C Only if you use the plot() function

D Only if you use ggplot2

You can specify exact coordinates like c(x, y) to position the legend precisely.

Explain how to add and customize a legend in a basic R plot.

Think about what you want the legend to show and where it should appear.

Describe the role of the 'pch' and 'col' arguments in the legend() function.

These arguments help the legend look like the plot points or lines.

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