Principles of Information Display for Visualization Practitioners

Note: a German translation by Jens Meiert is available here.

Introduction

Much of this paper is Tufte paraphrased; e.g., where Tufte might say `graphical excellence', I write `visualization excellence'. When you see the word `ink' (paper technology!), think `non-back-ground pixels'. Passages in quotation marks are direct quotes. Most of the text is a re-wording of Dr. Tufte's ideas, but all comments on the current state of visualization belong to me; and I am responsible for all errors.

The reader is encouraged to read Tufte's books. The treatment here is brief, incomplete, picture-poor, and low resolution.

Excellence

• "consists of complex ideas communicated with clarity, precision, and efficiency.
• is that which gives to the viewer the greatest number of ideas in the shortest time with the least ink in the smallest space.
• is nearly always multivariate.
• requires telling the truth about the data."

• "show the data
• induce the viewer to think about the substance rather than about methodology, graphic design, the technology..., or something else
• avoid distorting what the data have to say
• present many numbers in a small space
• make large data sets coherent
• encourage the eye to compare different pieces of data
• reveal the data at several levels of detail, from a broad overview to the fine structure
• serve a reasonably clear purpose: description, exploration, tabulation, or decoration
• be closely integrated with the statistical and verbal descriptions of a data set."

Principles

• content focus
• comparison rather than mere description
• integrity
• high resolution
• utilization of classic designs and concepts proven by time.

Content Focus

Assume that the viewer is just as smart as you and cares just as much. Never `dumb-down' a visualization.

Comparison vs. Description

To focus a visualization on "Compared to what?" enforce visual comparisons, particularly within the eyespan. Avoid relying on the viewer's memory to make visual comparisons; a weak facility in most of us.

Integrity

• "The representation of numbers, as physically measured on the surface of the graphic itself, should be directly proportional to the numerical quantities represented.
• Clear, detailed, and thorough labeling should be used to defeat graphical distortion and ambiguity.
• Write out explanations of the data on the graphic itself. Label important events in the data.
• Show data variation, not design variation.
• The number of information-carrying (variable) dimensions depicted should not exceed the number of dimensions in the data.
• Graphics must not quote data out of context."

High Resolution

• 8 Mbits - good PC screen
• 24 Mbits - high end workstation screen
• 25 Mbits - 35mm slide
• 150 Mbits - large topographic map

• 5-15 Kcharacters/page - non-fiction best sellers
• 10-18 Kcharacters/page - telephone books
• 28 Kcharacters/page - reference books

Classic Designs

Small Multiples

A small multiple design consists of a single design repeated several times within the eyespan, each example showing a different value of the independent variable(s). "Comparison must be enforced within the scope of the eyespan," a task at which small multiples excel. Thus, "for a wide range of problems in data presentation, small multiples are the best design solution." "At the heart of quantitative reasoning is a single question: Compared to what? Small multiple designs, multivariate and data bountiful, answer directly by visually enforcing comparisons of changes, of the differences among objects, of the scope of alternatives."

"Well designed small multiples are

• inevitably comparative
• deftly multivariate
• shrunken, high density graphics
• usually based on a single large data matrix
• drawn almost entirely with data-ink
• efficient in interpretation
• often narrative in content, showing shifts in the relationship between variables as the index variable changes (thereby revealing interaction or multiplicative effects)."

Small multiples are a straightforward extension to many current visualization systems, although graphics performance may be a problem.

Time Series

"The time-series plot is the most frequently used form of graphic design." One dimension, usually the horizontal, is time, and the graphics march along showing variation as time proceeds. Most visualization time-series works are videos, which show time by changing the picture, requiring the user to remember what came before. Finding innovative ways to incorporate time-series into visualization systems should be given serious consideration.

Micro/Macro Composition

Micro/macro composition refers to an approach where a visualization contains enormous detail, but an overall pattern emerges. "Panorama, vista, and prospect deliver to viewers the freedom of choice that derives from an overview, a capacity to compare and sift through detail. And that micro-information, like smaller texture in landscape perception, provides a credible refuge where the pace of visualization is condensed, slowed, and personalized."

Design Guidelines

• "have a properly chosen format and design
• use words, numbers, and drawing together
• reflect a balance, a proportion, a sense of relevant scale
• display an accessible complexity of detail
• often have a narrative quality, a story to tell about the data
• are drawn in a professional manner, with the technical details of production done with care
• avoid content-free decoration, including chartjunk"

Data-ink ratio

"Data-ink ratio = data-ink / total ink used to print the graphic = proportion of a graphic's ink devoted to the non-redundant display of data-information = 1.0 - proportion of a graphic that can be erased without loss of data-information"

One should

• "maximize the data-ink ratio, within reason"
• "erase non-data-ink, within reason"
• "erase redundant data-ink, within reason"

• "Above all else show the data.
• Maximize the data-ink ratio.
• Erase non-data-ink.
• Erase redundant data-ink.
• Revise and edit."

An interesting example of non-data ink erasure is range frames. The frame of an xy-plot is non-data ink. Most of the frame can be erased without loss of information, and if only the portion of the frame between minimum and maximum values is left, the frame provides additional information!

Clutter

Empty space may reduce clutter, but "it is not how much empty space there is, but rather how it is used. It is not how much information there is, but rather how effectively it is arranged."

Low density computer displays lead to spreading information out over many screens or dialog boxes. This leads to the "one damn thing after another" syndrome which causes users to get lost in an information maze. Place information adjacent in space, not stacked in time, to avoid the `Where am I?' problem.

Layering and Separation

1 + 1 = 3 or More

A particularly common example of 1 + 1 = 3 or more is boxes around text. "Unless deliberate obscurity is sought, avoid surrounding words by little boxes, which activate negative white spaces between word and box." Note that the box is non-data ink.

Color

• "Color spots against a light gray or muted field highlight and italicize data." "Note the effectiveness and elegance of small spots of intense, saturated color for carrying information."
• "use colors found in nature, especially those on the lighter side."
• "For encoding information,... more than 20 or 30 colors frequently produce not diminishing but negative returns."
• "The primary colors (yellow, red, blue) and black provides maximum differentiation (no four colors differ more)."
• In color maps, use a single hue, Don't use up the entire color spectrum, or even all of a hue's levels. Particularly avoid Roy G. Biv (red, orange, yellow, green, blue, indigo, violet), the color spectrum of the rainbow. It's good physics, but poor human factors. Like all multi-hue color maps, the non-equidistant hue changes are perceived as especially important contours, which they usually are not. Furthermore, the lighter middle parts of the spectrum are often perceived as the higher values. Finally, one needs to constantly remind oneself which color means high vs. low values. Using a single hue with variations in intensity allows instant interpretation, multiple color maps without ambiguity, and leaves graphical space for layering and separation.

• "Pure, bright or very strong colors have loud, unbearable effects when they stand unrelieved over large areas adjacent to each other, but extraordinary effects can be achieved when they are used sparingly on or between dull background tones."
• "The placing of light, bright colors mixed with white next to each other usually produces unpleasant results, especially if the colors are used for large areas."
• "Large area background or base-colors should do their work most quietly, allowing the smaller, bright areas to stand out most vividly, if the former are muted, grayish or neutral."
• "If a picture is composed of two or more large, enclosed areas in different colors, then the picture falls apart. Unity will be maintained, however, if the colors of one area are repeatedly intermingled in the other...."

Miscellaneous Admonitions

• "Above all, do no harm."
• Avoid codes, particularly one-time codes and legends. Place legends directly on the visualization.
• Use the smallest effective distance. "Make every visual move as small as possible, but keep clarity."
• "Mobilize every graphical element, perhaps several times over, to show the data."
• Tone down grids. Heavy, dominating grids often mar a graphic, overwhelming the data. Draw grids with a thin line and a light color, preferably grey. Let the data stand out.
• For a few numbers, use a table.
• "Transparent and effective deployment of redundant signals requires, first, the need--an ambiguity or confusion in seeing a data display that can in fact be diminished by multiplicity--and second, the appropriate choice of design technique."
• There are lots of good graphical designs. You needn't invent more. Research the literature, find good information designs, and steal them. Remember that talent imitates, genius steals.
• "Graphics should tend toward the horizontal, greater in length than in height. If the nature of the data suggests the shape of the graphic, follow that suggestion. Otherwise, move toward horizontal graphics about 50 percent wider than tall"
• Those that do the work should get the credit, sign your work.

Parting Shots

"Note all the different techniques for displaying sunspots during 380 years of data analysis -- from Galileo's first precious observation of the solar disks, to small multiple images, to dimensionality and data compression, and finally to micro/macro displays combining pattern and detail, average and variation. Exactly the same design strategies are found, again and again, in the work of those faced with a flood of data and images, as they scramble to reveal, within the limits of flatland, their detailed and complex information. These design strategies are surprisingly widespread, albeit little appreciated, and occur quite independently of the content of the data. "

References