What does width control?

In position_dodge(width = ...), width specifies the total dodging span (interval) on the x-axis into which all dodged elements for a given x-axis level are arranged side-by-side.

By default, if you do not explicitly supply width to position_dodge(), it inherits its value from the parent geom (e.g., geom_bar() defaults to width = 0.9).

Interaction between geom_bar(width) and position_dodge(width)

• geom_bar(width = ...): Sets the total physical width allocated to the bars at each x-axis level. The individual bar width is derived by dividing this value by the number of elements at that x-axis level.
• position_dodge(width = ...): Sets the total span on the x-axis across which the center positions of the dodged elements are calculated and spaced.

Both parameters use x-axis coordinate units (where the distance between adjacent discrete x-axis levels is 1.0).

Let’s examine how varying both parameters affects the layout using the mtcars dataset (plotting mean mpg across x-axis level cyl, with group variable gear mapped to fill):

# Base plot configuration
base_p <- ggplot(mtcars, aes(x = factor(cyl), y = mpg, fill = factor(gear))) +
  labs(x = "Cylinders (x-axis level)", y = "Mean MPG", fill = "Gear (group variable)") +
  theme_minimal(base_size = 11)

# Case 1: Default matching widths (0.9 / 0.9)
# Bars touch neatly within each x-axis level
p1 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    width = 0.9, 
    position = position_dodge(width = 0.9)
  ) +
  ggtitle("1. Matching widths (0.9 / 0.9)", subtitle = "Bars touch neatly within each x-axis level")

# Case 2: geom_bar width (0.5) < dodge width (0.9)
# Dodging span is 0.9, but bars are narrower -> gaps appear between bars
p2 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    width = 0.5, 
    position = position_dodge(width = 0.9)
  ) +
  ggtitle("2. geom_bar(0.5) < dodge(0.9)", subtitle = "Gaps created between dodged elements")

# Case 3: geom_bar width (0.9) > dodge width (0.5)
# Dodging span is narrower than bar widths -> bars overlap
p3 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    width = 0.9, 
    position = position_dodge(width = 0.5)
  ) +
  ggtitle("3. geom_bar(0.9) > dodge(0.5)", subtitle = "Dodged elements overlap")

# Case 4: Both changed to 0.6
# Group as a whole is narrower, bars touch, larger separation between x-axis levels
p4 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    width = 0.6, 
    position = position_dodge(width = 0.6)
  ) +
  ggtitle("4. Both set to 0.6", subtitle = "Compact cluster, wider space between x-axis levels")

(p1 | p2) / (p3 | p4)

As demonstrated: - Increasing width in geom_bar() increases the individual width of dodged elements at that x-axis level. - Increasing width in position_dodge() increases the total dodging span across which elements are spread. - When both values are equal, the dodged elements within each x-axis level touch each other without overlapping or leaving gaps.

Key Differences:

1. Boundary-Based Packing vs. Fixed Slot Offsets:
   • position_dodge() divides the total dodge width at an x-axis level into fixed slots and places each level of the group variable into its designated slot.
   • position_dodge2() packs elements (for different levels of the group variable) sequentially side-by-side using the bounding boxes of the geoms.
2. The width Parameter Has No Visual Effect in position_dodge2() with Bars:
   • Because position_dodge2() arranges bars based on their pre-computed bounding intervals rather than calculating center slot offsets from a dodge span, changing width in position_dodge2() produces no change in the plot output when bar widths are already defined by the parent geom.
   • Bar widths must be controlled directly via geom_bar(width = ...).
3. Built-in padding and reverse Support:
   • padding: Adds proportional space between dodged elements at the same x-axis level without requiring manual mismatch of geom and dodge widths.
   • reverse = TRUE: Reverses the left-to-right plotting order of the group variable without needing to alter factor levels.

Demo: width Has No Effect in position_dodge2()

Below, we compare position_dodge2(width = 0.3) against position_dodge2(width = 0.9). Notice that the two plots are completely identical:

# Changing width in position_dodge2 produces identical output
p_d2_w03 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    position = position_dodge2(width = 0.3)
  ) +
  ggtitle("position_dodge2(width = 0.3)", subtitle = "Width parameter has no visual effect")

p_d2_w09 <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    position = position_dodge2(width = 0.9)
  ) +
  ggtitle("position_dodge2(width = 0.9)", subtitle = "Identical layout to width = 0.3")

p_d2_w03 | p_d2_w09

Creating Spacing with padding and Reversing Order with reverse

Instead of altering width, position_dodge2() provides the padding and reverse parameters:

# Using padding and reverse in position_dodge2
p_dodge2_demo <- base_p +
  geom_bar(
    stat = "summary", 
    fun = "mean", 
    position = position_dodge2(padding = 0.2, reverse = TRUE)
  ) +
  labs(
    title = "position_dodge2(padding = 0.2, reverse = TRUE)",
    subtitle = "Built-in bar padding and reversed order of group variable"
  )

p_dodge2_demo