Encoding Values
The effective display of quantitative information involves two fundamental challenges
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Best medium (table/graph type).
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Designing graph to convey message.
Encoding Values in Graphs
Data graphics ~ verbal language
The rules of graphical communication are rarely arbitrary
Usually based on an understanding of visual perception:
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How we see,
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Visually encode information for easy and accurate decoding by audience.
Most Graphs Are …
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2D
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2 axes (horizontal, x; vertical, y)
Value-Encoding Objects
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Points
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Lines
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Bars
Points
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Pinpoint specific location on graph,
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Encode values associated with scale on each axis.
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Shapes (dots, squares), filled/open.
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YES: dot plot, strip plot, scatter plot.
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NO: time series.
Lines
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Lines connect points.
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Encode values by location.
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Ends of line segments mark values.
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Overall shape of data values.
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Slope is meaningful.
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YES: time series.
Points & Lines
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Overall shape & individual values.
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Multiple time series.
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Lines should only connect points that are related/connected.
Bars
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Encode values 2 ways:
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location of end point,
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length/height
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Easy to compare lengths to determine relative magnitude.
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Discrete values, not connected.
Bars
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Bars can be used for connected values (e.g., time series) when:
- you want to compare individual values at specific times, or
- histogram
Bars need to start at zero
In this case, points are better
Relationships
Time series
Values change through time (e.g., $ per mo.)
Ranking
Values ordered by size (e.g., sales, population)
Part-to-Whole
Values represent parts/proportions of a whole (e.g., relative cover, regional sales)
Deviation
Values represent the difference between two sets of values (e.g., income vs outgoing)
Distribution
Counts of values per interval/bin (e.g., number of trees per size class)
Correlation
Comparison of two paired sets of values (e.g., height vs weight)
Geospatial
Values displayed on a map (e.g., population per city, species richness per site)
Nominal Comparison
Values are compared for unordered categories (e.g., regions, fruit type)
Graph Selection Matrix
