Coordinate systems in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a coordinate system in R plotting?

A coordinate system in R plotting defines how data points are placed on the graph using axes, usually x and y, to show their position visually.

intermediate

What does the coord_cartesian() function do in ggplot2?

coord_cartesian() zooms in or limits the visible area of a plot without removing data points outside the limits.

beginner

How does coord_flip() change a plot?

coord_flip() swaps the x and y axes, turning vertical plots into horizontal ones or vice versa.

intermediate

What is the difference between coord_fixed() and default coordinates?

coord_fixed() fixes the aspect ratio so one unit on the x-axis is the same length as one unit on the y-axis, keeping shapes proportional.

beginner

Why use different coordinate systems in R plots?

Different coordinate systems help show data clearly by changing how axes behave, zooming in, flipping axes, or keeping proportions, making graphs easier to understand.

Which function in ggplot2 flips the x and y axes?

A coord_flip()

B coord_cartesian()

C coord_fixed()

D coord_polar()

coord_flip() swaps the x and y axes in a plot.

What does coord_cartesian(xlim = c(0, 10)) do?

A Flips the axes

B Zooms the plot to show x values between 0 and 10

C Removes data points outside 0 to 10 on x-axis

D Fixes aspect ratio

coord_cartesian() zooms in on the plot area but keeps all data points.

Which coordinate system keeps the aspect ratio fixed?

A coord_flip()

B coord_trans()

C coord_cartesian()

D coord_fixed()

coord_fixed() keeps the aspect ratio so units are equal on both axes.

What happens if you use coord_polar() in ggplot2?

A Plot zooms in

B Axes are flipped

C Plot is shown in polar coordinates (circle)

D Aspect ratio fixed

coord_polar() changes the plot to use polar coordinates, making circular plots.

Why might you use coord_cartesian() instead of setting axis limits directly?

A It zooms without removing data points

B It flips axes

C It changes colors

D It fixes aspect ratio

coord_cartesian() zooms in but keeps all data, unlike setting limits which can remove data.

Explain how different coordinate systems affect the appearance of a plot in R.

Think about how axes and shapes change visually.

Describe a situation where you would use coord_flip() in a plot.

Consider when vertical bars are hard to read.

Practice

1. Which function in R's ggplot2 package is used to flip the x and y axes on a plot?

easy

A. coord_flip()

B. coord_cartesian()

C. coord_polar()

D. coord_fixed()

Solution

1. Step 1: Understand coordinate functions in ggplot2

   The ggplot2 package provides several coordinate functions to control plot appearance.

2. Step 2: Identify the function that flips axes

   coord_flip() swaps the x and y axes, effectively flipping the plot.

3. Final Answer:

   coord_flip() -> Option A

4. Quick Check:

   Flip axes = coord_flip() [OK]

Hint: Flip axes with coord_flip() in ggplot2 [OK]

Common Mistakes:

• Confusing coord_flip() with coord_cartesian()
• Using coord_polar() which changes to polar coordinates
• Thinking coord_fixed() flips axes

2. Which of the following is the correct syntax to set fixed aspect ratio in ggplot2?

easy

A. coord_fixed_ratio(1)

B. coord_fixed(scale = 1)

C. coord_fixed(aspect = 1)

D. coord_fixed(ratio = 1)

Solution

1. Step 1: Recall the function for fixed aspect ratio

   coord_fixed() sets a fixed aspect ratio between x and y axes.

2. Step 2: Check correct argument name

   The argument to specify ratio is ratio, so coord_fixed(ratio = 1) is correct syntax.

3. Final Answer:

   coord_fixed(ratio = 1) -> Option D

4. Quick Check:

   Fixed ratio uses ratio= in coord_fixed() [OK]

Hint: Use ratio= argument in coord_fixed() for aspect ratio [OK]

Common Mistakes:

• Omitting argument name and just passing 1
• Using aspect= instead of ratio=
• Using a non-existent function coord_fixed_ratio()

3. What will be the effect of this code snippet in R using ggplot2?

ggplot(mtcars, aes(wt, mpg)) + geom_point() + coord_cartesian(xlim = c(2, 4), ylim = c(15, 30))

medium

A. Zooms the plot to show only points with weight between 2 and 4 and mpg between 15 and 30

B. Removes points outside the specified limits from the data

C. Flips the x and y axes

D. Changes the plot to polar coordinates

Solution

1. Step 1: Understand coord_cartesian() behavior

   coord_cartesian() zooms the plot to specified limits without removing data points.

2. Step 2: Analyze the limits given

   The x-axis is limited to 2 to 4 (weight), and y-axis to 15 to 30 (mpg), so plot zooms to this area.

3. Final Answer:

   Zooms the plot to show only points with weight between 2 and 4 and mpg between 15 and 30 -> Option A

4. Quick Check:

   coord_cartesian() zooms plot, does not remove data [OK]

Hint: coord_cartesian() zooms plot, keeps all data points [OK]

Common Mistakes:

• Thinking coord_cartesian() removes points outside limits
• Confusing with coord_flip() which flips axes
• Assuming coord_polar() is used here

4. Identify the error in this ggplot2 code snippet:

ggplot(mtcars, aes(wt, mpg)) + geom_point() + coord_flip(ratio = c(1, 5))

medium

A. geom_point() is missing required argument

B. coord_flip() does not accept ratio argument

C. ggplot() is missing data argument

D. coord_flip() should be replaced with coord_cartesian()

Solution

1. Step 1: Check arguments accepted by coord_flip()

   coord_flip() flips axes but does not accept ratio argument.

2. Step 2: Identify correct way to set aspect ratio when flipping

   Aspect ratio should be set using coord_fixed(), not inside coord_flip().

3. Final Answer:

   coord_flip() does not accept ratio argument -> Option B

4. Quick Check:

   coord_flip() has no ratio argument [OK]

Hint: coord_flip() does not accept ratio argument [OK]

Common Mistakes:

• Trying to set ratio inside coord_flip()
• Confusing coord_flip() with coord_cartesian()
• Assuming geom_point() needs extra arguments

5. You want to create a pie chart in ggplot2 using coord_polar(). Which coordinate system transformation should you apply before coord_polar() to make the pie chart correctly represent proportions?

hard

A. Use geom_point() with coord_polar()

B. Use geom_bar() with stat = 'identity'

C. Use geom_bar() with default settings and map fill to factor variable

D. Use geom_line() with coord_polar()

Solution

1. Step 1: Understand pie chart creation in ggplot2

   Pie charts are created by using bar charts with polar coordinates.

2. Step 2: Identify correct geom and mapping

   Using geom_bar() with default settings and mapping fill to a factor variable creates bars representing counts, which coord_polar() converts to pie slices.

3. Final Answer:

   Use geom_bar() with default settings and map fill to factor variable -> Option C

4. Quick Check:

   Pie chart = geom_bar() + coord_polar() with fill mapped [OK]

Hint: Pie charts use geom_bar() + coord_polar() with fill mapped [OK]

Common Mistakes:

• Using geom_point() or geom_line() which don't create bars
• Setting stat='identity' unnecessarily for counts
• Not mapping fill to a factor variable