Data Visualization with ggplot2

The Grammar of Graphics

ggplot2 maps data variables (aesthetics: x, y, color, size, shape) to geometric objects (geoms: points, lines, bars, boxes). Every plot starts with ggplot(data, aes(x=var, y=var)) then adds layers with +. geom_point() makes scatter plots; geom_line() line charts; geom_histogram() histograms; geom_col()/geom_bar() bar charts; geom_boxplot() box plots. Facets (facet_wrap, facet_grid) create small multiples.

Scales, Themes, and Labels

Scales control how data maps to visual properties: scale_color_brewer() uses ColorBrewer palettes (designed for clarity and accessibility), scale_x_log10() transforms axes, scale_fill_manual() sets custom colors. Themes control non-data elements: theme_minimal(), theme_classic(), theme_bw(). Labs(title, x, y, color) sets labels. theme() customizes individual elements: theme(legend.position='bottom').

Advanced ggplot2: Statistical Layers

stat_smooth() adds regression lines with confidence intervals. geom_violin() shows distribution shape. geom_density_2d() shows 2D density contours. geom_tile() makes heat maps. coord_flip() rotates axes for horizontal bar charts. patchwork library combines multiple plots. ggsave() exports to PDF, PNG, or SVG at any resolution — R produces print-ready graphics with no post-processing.

library(ggplot2)
library(dplyr)

# Scatter plot with regression line
ggplot(mtcars, aes(x = wt, y = mpg, color = factor(cyl))) +
  geom_point(size = 3, alpha = 0.8) +
  geom_smooth(method = 'lm', se = TRUE, aes(group = 1),
              color = 'black', linetype = 'dashed') +
  scale_color_brewer(palette = 'Set1', name = 'Cylinders') +
  labs(
    title    = 'Car Weight vs Fuel Efficiency',
    subtitle = 'Data: mtcars | Dashed line: linear fit',
    x        = 'Weight (1000 lbs)',
    y        = 'Miles per Gallon'
  ) +
  theme_minimal(base_size = 13)

# Save high-resolution
ggsave('mpg_plot.pdf', width = 8, height = 5, dpi = 300)

# Faceted bar chart
ggplot(diamonds, aes(x = cut, fill = clarity)) +
  geom_bar(position = 'fill') +
  facet_wrap(~color) +
  coord_flip() +
  labs(y = 'Proportion', title = 'Diamond Cut by Clarity and Color') +
  theme_bw()