ggplot2 Skill

R visualization with the grammar of graphics via ggplot2 4.0.x. Covers geoms (point, line, bar, histogram, boxplot, violin, ribbon, tile, text), aesthetics, scales, coordinates, facets, and themes. Extension packages: patchwork (multi-panel composition), ggrepel (non-overlapping text labels), ggridges (ridgeline density plots), ggdist (distribution visualization). Use when execution language is R and static publication-quality figures are needed. Python equivalent: plotnine (which ports ggplot2's grammar to Python). For interactive R charts, use plotly-r instead.

What is ggplot2?

ggplot2 is the grammar of graphics implementation for R:

• Declarative: Describe what you want, not how to draw it
• Layered: Build plots by adding components with +
• The original: plotnine (Python) is a port of ggplot2, not the reverse
• Publication-ready: Extensive themes and customization for polished output
• Extensible: Hundreds of extension packages (patchwork, ggrepel, ggridges, ggdist)

Version Notes: ggplot2 4.0.x (Breaking Changes from 3.x)

ggplot2 4.0.0 is a major release with significant breaking changes. Code written for 3.x may need updates.

Critical: linewidth vs size for Lines

The size aesthetic for line-based geoms was deprecated in 3.4.0. In 4.0.x:

• geom_line(size = 1) still works but throws a deprecation warning
• geom_bar(size = 1) / geom_col(size = 1) silently ignores size (fallback removed)
• Always use linewidth for lines, borders, and outlines
• size remains correct for points only (geom_point(size = 3))

# WRONG (deprecated / ignored in 4.0)
geom_line(size = 1)
geom_bar(size = 0.5)

# CORRECT (4.0+)
geom_line(linewidth = 1)
geom_bar(linewidth = 0.5)

Other 4.0.x Breaking Changes

Change	3.x Behavior	4.0.x Behavior
S7 internals	S3 classes	S7 classes (affects extension authors, not users)
coord_trans()	Primary name	Renamed to coord_transform(); coord_trans() still works
theme_set() etc.	Primary names	Renamed: set_theme(), get_theme(), update_theme(), replace_theme(); old names still work
geom_errorbarh()	Primary	Deprecated; use geom_errorbar(orientation = "y")
geom_violin(draw_quantiles)	Geom parameter	Deprecated; use geom_violin(quantiles = c(...), quantile.linetype = 1) — quantiles are hidden by default in 4.0, so quantile.linetype (a non-0 value) is required to display them
fatten argument	In boxplot/crossbar/pointrange	Deprecated
borders()	Active	Deprecated; use annotation_borders()
Pre-3.0 deprecations	Warnings	Now errors
Theme geom defaults	Via update_geom_defaults()	New theme(geom = element_geom(...)) for global defaults
Binning defaults	Old boundary selection	Better adherence to nbin argument; may change existing plots
mgcv, tibble	Imported	Moved to Suggests upstream; both pre-installed in DAAF (no action needed)

New 4.0.x Features

• theme(geom = element_geom(...)) for global geom aesthetic defaults
• from_theme() inside aes() to reference theme defaults
• stat_connect() and stat_manual() new stats
• theme(panel.widths, panel.heights) for panel sizing
• labs(dictionary = ...) for label mapping by variable name
• ggsave() can write multi-page PDFs from a list of plots
• theme_*(ink, paper, accent) arguments for foreground/background/highlight colors

How to Use This Skill

Reference File Structure

File	Purpose	When to Read
quickstart.md	Basic ggplot pattern, ggsave, essential setup	Starting out or quick reminder
geoms.md	All major geoms with examples	Choosing chart types
scales.md	Scales, axes, color palettes, labels, the scales package	Axis/color/label formatting
facets.md	facet_wrap, facet_grid, labellers, spacing	Multi-panel layouts
themes.md	Built-in themes, custom theme(), publication-ready styling	Styling and polish
extensions.md	patchwork, ggrepel, ggridges, ggdist	Multi-panel, labels, distributions
gotchas.md	4.0 migration, common mistakes, factor ordering, save tips	Debugging or reviewing

Reading Order

1. Quick plot? Start with quickstart.md
2. Which geom? Check geoms.md
3. Customize scales/axes? Read scales.md
4. Multi-panel? Read facets.md
5. Publication polish? Read themes.md
6. Extensions? Read extensions.md
7. Trouble? Check gotchas.md

Related Skills

Skill	Relationship
plotnine	Python equivalent (plotnine ports ggplot2 to Python)
tidyverse	Data preparation -- tidy data feeds into ggplot2 pipelines
plotly-r	Interactive R charts (use when interactivity needed)
gt	Publication-quality tables (use for tabular output, not charts)
r-python-translation	Cross-language visualization translation
data-scientist	Method selection and visualization design guidance

Quick Decision Trees

"What chart type do I need?"

What are you visualizing?
├─ Relationship (x vs y)
│   ├─ Continuous x, continuous y → geom_point() + geom_smooth()
│   ├─ Time series → geom_line()
│   └─ With error/uncertainty → geom_pointrange() or geom_ribbon()
├─ Distribution
│   ├─ One variable → geom_histogram() or geom_density()
│   ├─ By group (few groups) → geom_boxplot() or geom_violin()
│   ├─ By group (many groups) → ggridges::geom_density_ridges()
│   └─ Full distribution detail → ggdist::stat_halfeye()
├─ Comparison
│   ├─ Counts → geom_bar()
│   ├─ Values → geom_col()
│   └─ Grouped → geom_col(position = "dodge")
├─ Composition
│   ├─ Parts of whole → geom_col(position = "fill")
│   └─ Over time → geom_area(position = "stack")
├─ Heatmap / tile → geom_tile() or geom_raster()
└─ Multiple plots → patchwork (p1 + p2) / p3

"How do I save this plot?"

Saving a plot?
├─ To PNG (default for DAAF) → ggsave("file.png", p, width = 10, height = 8, dpi = 300)
├─ To PDF → ggsave("file.pdf", p, width = 10, height = 8)
├─ To SVG → ggsave("file.svg", p, width = 10, height = 8)
├─ Multiple plots to one PDF → ggsave("file.pdf", list(p1, p2, p3))
└─ Temp file (smoke tests) → ggsave(tempfile(fileext = ".png"), p)

File-First Execution in Research Workflows

In DAAF research pipelines, all visualizations are generated through script files in scripts/stage8_analysis/, not interactively. This ensures auditability and reproducibility.

The pattern:

1. Write plot code to scripts/stage8_analysis/{step}_{plot-name}.R
2. Execute via bash {BASE_DIR}/scripts/run_with_capture.sh {script_path}
3. Output gets appended to the script as comments
4. Use ggsave() to save plots to the project output directory

See agent_reference/SCRIPT_EXECUTION_REFERENCE.md for the mandatory file-first execution protocol.

Quick Reference

Essential Setup

library(ggplot2)
library(scales)       # label_comma(), label_percent(), etc.
library(patchwork)    # plot composition: p1 + p2, p1 / p2
library(ggrepel)      # geom_text_repel(), geom_label_repel()
library(ggridges)     # geom_density_ridges()
library(ggdist)       # stat_halfeye(), stat_dots()

Basic Plot Pattern

p <- ggplot(df, aes(x = col_x, y = col_y)) +
  geom_point() +
  labs(title = "Title", x = "X Label", y = "Y Label") +
  theme_minimal()

ggsave("output.png", p, width = 10, height = 8, dpi = 300)

Common Geoms

Geom	Use Case
geom_point()	Scatter plots
geom_line()	Line plots / time series
geom_bar()	Count bars (stat = "count")
geom_col()	Value bars (stat = "identity")
geom_histogram()	Distributions
geom_density()	Density curves
geom_boxplot()	Box-and-whisker
geom_violin()	Violin plots
geom_smooth()	Trend lines
geom_tile()	Heatmaps

Common Aesthetics

Aesthetic	Controls	Use size or linewidth?
x, y	Position	N/A
color	Point/line color	N/A
fill	Area fill color	N/A
size	Point size only (4.0+)	size for points
linewidth	Line width (4.0+)	linewidth for lines
shape	Point shape	N/A
alpha	Transparency	N/A
linetype	Line pattern	N/A

Topic Index

Topic	Reference File
Basic plot pattern	./references/quickstart.md
ggsave() parameters	./references/quickstart.md
Data requirements	./references/quickstart.md
Scatter, line, bar, area	./references/geoms.md
Histogram, density, boxplot	./references/geoms.md
Smoothing, error bars	./references/geoms.md
Heatmaps, tiles, text	./references/geoms.md
Position adjustments	./references/geoms.md
Continuous/discrete scales	./references/scales.md
Color palettes (Brewer, viridis)	./references/scales.md
Axis labels and formatting	./references/scales.md
scales package helpers	./references/scales.md
facet_wrap, facet_grid	./references/facets.md
Free scales, labellers	./references/facets.md
Built-in themes	./references/themes.md
Custom theme() elements	./references/themes.md
Publication-ready themes	./references/themes.md
patchwork composition	./references/extensions.md
ggrepel labels	./references/extensions.md
ggridges ridgeline plots	./references/extensions.md
ggdist distribution viz	./references/extensions.md
linewidth vs size migration	./references/gotchas.md
4.0 breaking changes	./references/gotchas.md
Factor ordering	./references/gotchas.md
Save resolution/dimensions	./references/gotchas.md
Common errors	./references/gotchas.md

Citation

When ggplot2 is used as a primary visualization tool, include in the report's Software & Tools references:

Wickham, H. (2016). ggplot2: Elegant Graphics for Data Analysis (2nd ed.). Springer-Verlag New York. https://ggplot2.tidyverse.org

Cite when: ggplot2 produces figures included in the report. Do not cite when: Only used for quick exploratory plots not included in deliverables.