Grammar of Graphics with Altair

CAPP 30239

Today

• Grammar of Graphics
• Types of Data
• Intro to Altair

Grammar of Graphics

Hadley Wickham, creator of ggplot2 and tidyverse, "A Layered Grammar of Graphics".

Key Idea: move beyond pre-defined composites like "scatter plot" and "bar chart" into a composable grammar from which we can construct a wide variety of visualizations.

Wickham's Components:

1. data and aesthetic mappings,

2. one or more layers, each with

   • a geometric object (line, point, etc.)
   • (optional) statistical transformation
   • (optional) position adjustment

3. one scale per aesthetic mapping (color, size, etc.)

4. a coordinate system

5. facet specification

Types of Data

N - Nominal

"strings" with no order (alphabetical does not count)

Species
States
Countries

O - Ordered

• Grades: A, B, C, D, E, F
• Rankings: 1st, 2nd, 3rd

Types of Data (Quantitative)

Q - Interval (arbitrary zero)

• Dates (1 CE, Jan 1 1970, or...)
• Location (lat, lon)

Only differences matter, can't compare ratios.
(What is 2024 / 1990?)

Temporal

Some systems (like Altair) will also offer this option specifically for dates and times.

Q - Ratio (zero fixed)

Physical measurements, counts, amounts.

"4 km is twice as far as 2 km"

Types of Data (Operations)

Data Model to N, O, Q

• string?
• bool?
• float/int?

Possible exceptions?

Data Model to N, O, Q

Typically:

• string - nominal or ordered
• bool - nominal
• float/int - interval or ratio

Possible Exceptions?

• Numeric IDs
• ZIP Codes
• ratio data stored with units (e.g. "10km")

Mapping of Variables to Aesthetics

• position (X, Y, Z)
• length
• angle
• slope
• area
• volume
• density
• hue
• saturation
• texture
• connection
• containment/grouping
• shape

Mackinlay's "effectiveness"

Altair's Grammar

Altair condenses several of the different pieces of the grammar to "encoding channels".

We've seen X, Y, and color, let's take a look at some examples of other encoding channels.