VI. Viz Idioms for tabular data

Keys and Values

Key:
- Independent attributes
- Used as unique index to look up items
- Simple tables: 1 keys
- Multidimensional tables: multiple keys
Value:
- dependent attribute, value of cell
Classify arrangements by key count

0 Keys = Express Values

Idiom: Scatterplots - No keys, only values

Idiom	Scatterplots
What: Data	0 keys
Values = 2 quantitative attributes
How: Encode	Marks = Lines
Channels = horizontal and vertical position
→ Additional channels: color, size (bubbleplots)
Why: Task	Find trends, outliers, distribution, correlation; locate clusters
Scale	Items: hundred

1 or more keys = Separate, Order, and Align - Categorical Regions

Idiom: Bar Chart - one key, one value

Idiom	Bar Chart
What: Data	1 key = 1 categorial attribute
1 value = 1 quantitative attribute
How: Encode	Marks = Lines
Channels:

Express value attribute with aligned vertical position
Seperate key attribute with horizontal position - Ordered by quantitative attribute: by label (alphabetical), by length attribute (data-driven) | | Why: Task | Compare, lookup values | | Scale | Dozens to hundreds of levels for key attribute |

Idion: Stacked bar chart - one more key

Idiom	Stacked Bar Chart
What: Data	2 categorial attribute
1 quantitative attribute
How: Encode	Marks = Vertical Stack of Line marks
Glyph = Composite Object, Internal structure from multiple marks
Channels:

Length and color hue
Spatial Regions: one per glyph | | Why: Task | part-to-whole relationship | | Scale | several to one dozen levels for stacked attributes |

Idiom: Dot plot / line chart - one key, one value

Idiom	Dot plot / Line chart
What: Data	2 quantitative attribute
How: Encode	Marks = Points / Line connection between them
Channels:

Aligned length to express quantitative value
Seperated and Ordered by key attribute in to horizontal regions | | Why: Task | Find trend | | Scale | Hundred of key levels, hundred of value values |

Idiom: Dual-Axis Line Chart

Controversial

Acceptable if commensurate
Beware, very easy to mislead

Idiom: Indexed line charts - one key, one value

Idiom	Indexed Line chart
What: Data	2 quantitative attributes
Derived data: new quantitative value attribute
How: Encode
(same as standard line chart)
Marks = Points / Line connection between them
Channels:

Aligned length to express quantitative value
Seperated and Ordered by key attribute in to horizontal regions | | Why: Task | Show change over time Principle: Normalized, not absolute | | Scale | (same as standard line chart) Hundred of key levels, hundred of value values |

Choosing bar vs line charts = “Natural” Use

Depends on type of key attribute

Bar charts if categorical
Line charts if ordered