ggplot2: Modified Plot

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theme() function

The theme() function in ggplot2 allows you to customize various aspects of a plot’s appearance. Below is a categorized list of arguments in theme():

1. Text Elements

These control text appearance in different parts of the plot.

  • title: Title of the plot.

  • plot.title: Main plot title.

  • plot.subtitle: Subtitle of the plot.

  • plot.caption: Caption (text at the bottom of the plot).

  • axis.title: Titles of both axes.

    • axis.title.x

    • axis.title.y

  • axis.text: Labels of the axes.

    • axis.text.x

    • axis.text.y

  • legend.title: Legend title.

  • legend.text: Legend labels.

  • strip.text: Facet labels (for faceted plots).

    • strip.text.x

    • strip.text.y

element_text()

The element_text() function in ggplot2 is used within the theme() function to customize text elements like titles, axis labels, and legend text.

The list of arguments:

  1. Font Size & Style

  • family → Font family (e.g., "serif", "sans", "mono")

  • face → Font style

    • "plain" (default)

    • "bold"

    • "italic"

    • "bold.italic"

  • size → Font size (in points)

  • lineheight → Line spacing (multiplier, default is 1.2)

  1. Color & Transparency

  • color → Text color (e.g., "red", "blue", "#123456")

  • hjust → Horizontal justification (0 = left, 0.5 = center, 1 = right)

  • vjust → Vertical justification (0 = bottom, 0.5 = middle, 1 = top)

  • alpha → Transparency (0 = fully transparent, 1 = fully opaque)

  1. Position Adjustments

  • angle → Rotate text (degrees, e.g., 45, 90, -45)

  • margin → Add margins around text (margin(top, right, bottom, left))

Summary

Argument Description Example
family Font family (e.g., "serif", "sans") family = "serif"
face Font style ("bold", "italic", etc.) face = "bold"
size Font size (in points) size = 14
lineheight Line spacing lineheight = 1.5
color Text color color = "red"
alpha Transparency (0 to 1) alpha = 0.7
hjust Horizontal justification (0 to 1) hjust = 0.5
vjust Vertical justification (0 to 1) vjust = 1
angle Rotate text (degrees) angle = 45
margin Margin around text margin(5,5,5,5)

Summary of element_text() Usage in theme()

theme() Argument Description
plot.title Main title
plot.subtitle Subtitle
plot.caption Caption at bottom
plot.tag Tag (e.g., “A”, “B”)
axis.title.x X-axis title
axis.title.y Y-axis title
axis.text.x X-axis tick labels
axis.text.y Y-axis tick labels
legend.title Legend title
legend.text Legend labels
strip.text.x Facet labels (horizontal)
strip.text.y Facet labels (vertical) 
library(ggplot2)
mtcars |> 
ggplot() +
  aes( x= mpg, y = wt, color = factor(cyl)) +
  geom_point() +
  facet_grid(.~cyl) +
  theme(
    # Plot title, subtitle, caption, and tag
    plot.title = element_text(size = 18, face = "bold", color = "blue", hjust = 0.5),
    plot.subtitle = element_text(size = 14, face = "italic", color = "darkblue"),
    plot.caption = element_text(size = 10, color = "gray50"),
    plot.tag = element_text(size = 16, face = "bold", color = "red"),
    
    # Axis titles
    axis.title.x = element_text(size = 14, face = "bold", color = "purple"),
    axis.title.y = element_text(size = 14, face = "bold", color = "purple"),
    
    # Axis text (tick labels)
    axis.text.x = element_text(size = 12, color = "black", angle = 45, vjust = 1),
    axis.text.y = element_text(size = 12, color = "black"),
    
    # Legend title and text
    legend.title = element_text(size = 14, face = "bold", color = "darkred"),
    legend.text = element_text(size = 12, color = "black"),
    
    # Facet strip text
    strip.text.x = element_text(size = 14, face = "bold", color = "darkgreen"),
    strip.text.y = element_text(size = 14, face = "bold", color = "darkgreen")
  ) +
  labs(
    title = "Relationship between MPG and Weight",
    subtitle = "Colored by Number of Cylinders",
    caption = "Source: mtcars dataset",
    tag = "A"
  )

2. Background & Panel Elements

These control the background, grid, and panel appearance.

  • plot.background: Background of the entire plot.

  • panel.background: Background of the plotting area.

  • panel.grid: Grid lines.

    • panel.grid.major

    • panel.grid.minor

  • panel.border: Border around the plot panel.

  • panel.spacing: Spacing between facets.

3. Axis Customization

These control the appearance of axes and ticks.

  • axis.line: Axis lines.

  • axis.ticks: Ticks on the axes.

    • axis.ticks.x

    • axis.ticks.y

  • axis.text: Axis labels.

    • axis.text.x

    • axis.text.y

  • axis.title: Axis titles.

    • axis.title.x

    • axis.title.y

4. Legend Elements

These control the appearance of the legend.

  • legend.background: Background of the legend.

  • legend.key: Background of legend keys.

  • legend.key.size: Size of legend keys.

  • legend.position: Position of the legend.

    • "none", "left", "right", "top", "bottom"

  • legend.direction: Layout direction ("horizontal" or "vertical").

  • legend.box: Arrangement of multiple legends ("horizontal" or "vertical").

5. Facet Customization

These control the appearance of facet labels.

  • strip.background: Background color of facet labels.

  • strip.text: Text of facet labels.

    • strip.text.x

    • strip.text.y

  • strip.placement: Placement of facet labels ("inside" or "outside").

6. Margins & Spacing

These control spacing between different elements.

  • plot.margin: Margins around the plot.

  • legend.margin: Margins around the legend.

element_rect() function

The element_rect() function in ggplot2 is used within the theme() function to customize rectangular elements like backgrounds, borders, and legend keys.

The list of available arguments:

1. Fill & Border Color

  • fill → Background color (e.g., "red", "blue", "#123456", NA for transparent)

  • colour / color → Border color (same format as fill)

2. Border Line Settings

  • linewidth → Border line thickness (default 0.5)

  • linetype → Border line type:

    • 1 or "solid" → Solid line

    • 2 or "dashed" → Dashed line

    • 3 or "dotted" → Dotted line

    • 4 or "dotdash" → Dot-dash line

    • 5 or "longdash" → Long dash

    • 6 or "twodash" → Two dashes

3. Rounded Corners

  • r → Corner radius (default 0, set higher for rounded corners)

Summary of element_rect() Arguments

Argument Description Example
fill Background color fill = "gray90"
color / colour Border color colour = "black"
linewidth Border thickness linewidth = 1.5
linetype Border line type linetype = "dashed"
r Corner radius (rounded corners) r = 5

Summary of element_rect() Usage in theme()

theme() Argument Description
plot.background Entire plot background
panel.background Plotting area background
panel.border Border around the plotting area
legend.background Background behind the legend
legend.box.background Border around the legend box
legend.key Background of legend keys
strip.background Background of facet labels 
mtcars |> 
ggplot() +
  aes(x = mpg, y = wt, color = factor(cyl)) +
  geom_point(size = 3) +
  facet_grid(.~cyl) +
  theme(
    # Background settings
    plot.background = element_rect(fill = "gray90", color = "black", linewidth = 2),
    panel.background = element_rect(fill = "white", color = "black", linewidth = 1),
    
    # Borders
    panel.border = element_rect(color = "red", fill = NA, linewidth = 1.5),
    
    # Legend
    legend.background = element_rect(fill = "lightblue", color = "black", linewidth = 1),
    legend.box.background = element_rect(fill = "gray80", color = "black", linewidth = 1.5),
    legend.key = element_rect(fill = "white", color = "black", linewidth = 0.5),
    
    # Facet strip (for facet_wrap)
    strip.background = element_rect(fill = "yellow", color = "black", linewidth = 1)
  ) +
  labs(
    title = "MPG vs Weight with Custom Theme",
    subtitle = "Customized with element_rect()"
  )

element_line() function

The element_line() function in ggplot2 is used within theme() to customize lines in a plot, such as axis lines, grid lines, and borders.

Arguments in element_line()

The list of available arguments:

1. Line Appearance

  • colour / color → Line color (e.g., "black", "red", "#123456")

  • size → Line thickness (default is 0.5)

  • linetype → Line type:

    • 1 or "solid" → Solid line

    • 2 or "dashed" → Dashed line

    • 3 or "dotted" → Dotted line

    • 4 or "dotdash" → Dot-dash line

    • 5 or "longdash" → Long dash

    • 6 or "twodash" → Two dashes

  • lineend → Line end style:

    • "round" (default)

    • "butt"

    • "square"

2. Transparency

  • alpha → Transparency (0 = fully transparent, 1 = fully opaque)

Summary of element_line() Arguments

Argument Description Example
colour / color Line color color = "red"
size Line thickness size = 1
linetype Line type (solid, dashed, etc.) linetype = "dotted"
lineend Line end style (round, butt, square) lineend = "round"
alpha Transparency (0 to 1) alpha = 0.7

Summary of element_line() Usage in theme()

theme() Argument Description
axis.line.x Line along the x-axis
axis.line.y Line along the y-axis
panel.grid.major.x Major grid lines along the x-axis
panel.grid.major.y Major grid lines along the y-axis
panel.grid.minor.x Minor grid lines along the x-axis
panel.grid.minor.y Minor grid lines along the y-axis
strip.switch.pad.grid Line between facet strips (grid layout)
strip.switch.pad.wrap Line between wrapped facets 
mtcars |> 
ggplot() +
  aes(mpg, wt, color = factor(cyl)) +
  geom_point(size = 3) +
  theme(
    # Axis lines
    axis.line.x = element_line(color = "black", linewidth = 1, linetype = "solid"),
    axis.line.y = element_line(color = "black", linewidth = 1, linetype = "solid"),
    
    # Major grid lines
    panel.grid.major.x = element_line(color = "gray", linewidth = 0.5, linetype = "dashed"),
    panel.grid.major.y = element_line(color = "gray", linewidth = 0.5, linetype = "dotted"),
    
    # Minor grid lines
    panel.grid.minor.x = element_line(color = "lightgray", linewidth = 0.3, linetype = "dashed"),
    panel.grid.minor.y = element_line(color = "lightgray", linewidth = 0.3, linetype = "dotted"),
    

    # Facet strip separators
    strip.switch.pad.grid = unit(0.5, "cm"),
    strip.switch.pad.wrap = unit(0.5, "cm")
  ) +
  labs(
    title = "MPG vs Weight with Custom Line Styling",
    subtitle = "Customized with element_line()"
  )

element_blank() function

The element_blank() function in ggplot2 is used within theme() to completely remove an element from a plot.

  • element_blank() removes an element completely.

  • Useful for minimalist plots.

  • Works inside theme() with axis labels, titles, grid lines, legends, backgrounds, etc..

Summary of element_blank() Usage in theme()

theme() Argument Description
axis.text.x / axis.text.y Remove axis text labels
axis.title.x / axis.title.y Remove axis titles
axis.line.x / axis.line.y Remove axis lines
axis.ticks.x / axis.ticks.y Remove axis ticks
panel.grid.major / panel.grid.minor Remove grid lines
plot.background Remove the entire plot background
panel.background Remove the background of the plotting area
panel.border Remove the border around the plot panel
legend.background Remove the legend background
legend.key Remove the background of legend items
legend.title Remove the legend title
legend.text Remove the legend text
strip.background Remove the background of facet labels
strip.text Remove the text inside facet labels

annotae() function

The annotate() function in ggplot2 allows you to add text, shapes, or other graphical elements to a plot. It is particularly useful for adding annotations that are not tied to data points but rather placed at specific coordinates.

Syntax of annotate()

annotate(geom, 
         x, y, label = NULL, 
         color = "black", size = 5, ...)

  • geom → Type of annotation ("text", "label", "rect", "segment", "point", etc.).

  • x, y → Position coordinates for the annotation.

  • label → Text to be displayed (if geom = "text" or geom = "label").

  • color → Color of the annotation.

  • size → Size of text or shape.

  • Additional parameters depending on the geom.

Examples of annotate() Usage

1️) Adding Text Annotation

library(ggplot2)
mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "text", 
           x = 25, y = 4, 
           label = "Fuel Efficient", color = "red", size = 5)

🔹 Adds a red text label at (x = 25, y = 4).

2️) Adding a Label with a Background

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "label", 
           x = 15, y = 5, 
           label = "Heavy Car", fill = "yellow", 
           color = "black", size = 5)

🔹 Adds a label with a yellow background.

3️) Adding a Rectangle

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "rect", 
           xmin = 20, xmax = 30, ymin = 2, ymax = 4, 
           alpha = 0.2, 
           fill = "blue")

🔹 Draws a semi-transparent blue rectangle.

4️) Adding a Segment (line, arrow)

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "segment", 
           x = 10, xend = 30, y = 5, yend = 2, 
           color = "blue", size = 1, linetype = "dashed")
Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
ℹ Please use `linewidth` instead.

🔹 Adds a dashed blue line from (10, 5) to (30, 2).

Adding an Arrow Annotation

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "segment", 
    x = 15, xend = 25,  # Start and end X coordinates
    y = 5, yend = 3,    # Start and end Y coordinates
    color = "red", 
    size = 1, 
    arrow = arrow(type = "closed", length = unit(0.3, "cm")) # Arrow style
  )

🔹 This adds a red arrow pointing from (15,5) to (25,3).

Customize the Arrow

We can change:

  • Arrow type: "open", "closed", or "both"

  • Arrow length: unit(0.5, "cm")

  • Line type: linetype = "dashed"

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "segment", 
    x = 10, xend = 30, 
    y = 4, yend = 2, 
    color = "blue", 
    size = 1, 
    linetype = "dashed",
    arrow = arrow(type = "open", length = unit(0.5, "cm"))
  )

🔹 This adds a dashed blue arrow with an open arrowhead.

5️) Adding a Point

mtcars |> 
ggplot() +
  aes(x = mpg, y = wt) +
  geom_point() +
  annotate(geom = "point", 
           x = 25, y = 3, 
           color = "red", size = 5, shape = 8)

🔹 Adds a red star-shaped point at (25,3).

When to Use annotate()?

✅ When you want to add static text or shapes that are not dependent on data.

✅ When you want to highlight specific areas of a plot.

✅ When you need annotations that stay in place even if the data changes.