Legend control in R Programming - Step-by-Step Execution

Concept Flow - Legend control

Create plot object

↓

Add data layers

↓

Add legend control

↓

Customize legend appearance

↓

Render plot with legend

This flow shows how to build a plot step-by-step, add a legend control, customize it, and then display the final plot.

Execution Sample

R Programming

library(ggplot2)

p <- ggplot(mtcars, aes(x=wt, y=mpg, color=factor(cyl))) +
  geom_point() +
  labs(color='Cylinders')

print(p)

This code creates a scatter plot of car weight vs. mileage, colors points by cylinder count, adds a legend title, and displays the plot.

Execution Table

Step	Action	Evaluation	Result
1	Load ggplot2 library	library(ggplot2)	ggplot2 functions available
2	Create ggplot object with mtcars data	ggplot(mtcars, aes(x=wt, y=mpg, color=factor(cyl)))	Plot object with aesthetics set
3	Add points layer	geom_point()	Points layer added to plot
4	Add legend label for color	labs(color='Cylinders')	Legend title set to 'Cylinders'
5	Print plot	print(p)	Plot rendered with legend showing cylinder groups
6	End	No more commands	Execution stops

💡 All plot components added and plot displayed with legend control

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
p	NULL	ggplot object with aesthetics	ggplot + points layer	ggplot + points + legend label	Final plot object ready to render

Key Moments - 3 Insights

Why does the legend appear automatically without extra code?

Because ggplot2 automatically creates legends for mapped aesthetics like color, as shown in execution_table step 2 and 3.

How do we change the legend title?

By using labs(color='Cylinders') as in step 4, which sets the legend label for the color aesthetic.

What happens if we don't call print(p)?

The plot won't display in some environments; print(p) in step 5 triggers rendering.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of variable 'p' after step 3?

A Plot object with legend label set

B Plot object with points layer added

C NULL

D Plot rendered on screen

💡 Hint

Check variable_tracker column 'After Step 3' for 'p'

At which step is the legend title set to 'Cylinders'?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look at execution_table row with action 'Add legend label for color'

If we skip step 5 (print(p)), what happens?

A Plot object is created but not displayed

B Error occurs

C Plot is rendered anyway

D Legend disappears

💡 Hint

Refer to key_moments answer about print(p) necessity

Concept Snapshot

Legend control in R ggplot2:
- Map aesthetics like color to data for automatic legends
- Use labs(color='Title') to set legend title
- Add layers (geom_point) to build plot
- Call print() to render plot with legend
- Legend updates automatically with mapped aesthetics

Full Transcript

This example shows how to control legends in R using ggplot2. First, we load the library. Then, we create a plot object mapping weight and mileage, coloring points by cylinder count. Adding geom_point adds the data points. Using labs(color='Cylinders') sets the legend title. Finally, print(p) renders the plot with the legend visible. The legend appears automatically because ggplot2 creates legends for mapped aesthetics. The legend title is customized with labs(). Without print(), the plot won't display in some environments. This step-by-step trace helps understand how legend control works in R plotting.

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