A Detailed Look at Grammar of Graphics

EE BIOL C177/C234

Chuliang Song

Today’s Menu 🎯

1. Aesthetic mappings — inside vs outside aes()
2. Geometric objects — geom_*() functions
3. Global vs local organization

Aesthetic Mappings

Inside vs Outside aes()

❌ Inside aes() — wrong for fixed values

Creates a legend with “blue” as a label!

✅ Outside aes() — correct for fixed values

All points become blue

Another Example: size

✅ Inside aes() — map a variable

Size varies by body mass

✅ Outside aes() — set a fixed value

All points are the same size

The Rule

Inside aes() = map a variable from your data

Outside aes() = set a fixed value for all points

Common Aesthetics

Aesthetic	Description	Example
x, y	Position	aes(x = bill_length_mm)
color	Color	aes(color = species)
shape	Point shape	aes(shape = island)
size	Point size	aes(size = body_mass_g)
alpha	Transparency	alpha = 0.5

Combining Aesthetics

Exercise

Fix the aesthetic mapping error:

Geometric Objects

Same Data, Different Geom

Layering Geoms

Order matters — last layer is on top!

Global vs Local

Global Aesthetics

Defined in ggplot() → apply to all layers

Local Aesthetics

Defined in geom_*() → apply to that layer only

Best Practice

• Put shared aesthetics in ggplot()
• Put layer-specific aesthetics in geom_*()
• When unsure → put it in geom_*()

Local Data Too!

You can even use different data per layer:

Summary

• aes() maps variables → visual properties
• Fixed values go outside aes()
• Different geom_*() = different visualizations
• Global (in ggplot()) vs local (in geom_*())