2.5 KiB
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:
-
data and aesthetic mappings,
-
one or more layers, each with
- a geometric object (line, point, etc.)
- (optional) statistical transformation
- (optional) position adjustment
-
one scale per aesthetic mapping (color, size, etc.)
-
a coordinate system
-
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)
| =, != | <, >, <=, >= | +, - | ÷ | |
|---|---|---|---|---|
| Nominal | ✓ | |||
| Ordered | ✓ | ✓ | ||
| Interval | ✓ | ✓ | ✓ | |
| Ratio | ✓ | ✓ | ✓ | ✓ |
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.