Introduction to Data Analytics

Part Four: Principles of Data Visualization

David Schuff

What makes a good chart?

Minard’s map of Napoleon’s campaign into Russia, 1869Reprinted in Tufte (2009), p. 41

What makes a good chart?

http://www.popvssoda.com/countystats/total-county.html

What makes a good chart?

Zhang et al. (2010), “A case study of micro-blogging in the enterprise: use, value, and related issues,” Proceedings of the 28th International Conference on Human Factors in Computing Systems.

This is from an academic conference

paper.

What are the problems with

this chart?

Some basic principles (adapted from Tufte 2009)

• The chart should tell a story1• The chart should have graphical

integrity2• The chart should minimize

graphical complexity3Tufte’s fundamental principle:Above all else show the data

Principle 1: The chart should tell a story

Graphics should be clear on their own

The depictions should enable meaningful comparison

The chart should yield insight beyond the text

“If the statistics are boring, then you’ve got the wrong numbers.” (Tufte 2009)

Examples?http://www.evl.uic.edu/aej/491/week03.html

http://flowingdata.com/2009/11/26/fox-news-makes-the-best-pie-chart-ever/

Telling a Story

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Principle 2: The chart should have graphical integrity

• Basically, it shouldn’t “lie” (mislead the reader)

• Tufte’s “Lie Factor”:

Should be ~ 1

< 1 = understated effect

> 1 = exaggerated effect

Examples of the “lie factor”

𝐿𝐹=5.3 /0.627.5 /18

=8.831.53

=5.77

𝐿𝐹=4280% ( h𝑐 𝑎𝑛𝑔𝑒𝑖𝑛𝑣𝑜𝑙𝑢𝑚𝑒)454% ( h𝑐 𝑎𝑛𝑔𝑒 𝑖𝑛𝑝𝑟𝑖𝑐𝑒)

=9.4Reprinted from Tufte (2009), p. 57 & p. 62

A more recent, basic example

http://20bits.com/articles/politics-and-tuftes-lie-factor/

The original graphic from Real Clear Politics, 2008.

(Look at the y-axis)The adjusted graphic.

Other tips to avoid “lying”

Adjust for inflation

Make sure the context is presented

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2003 2004 2005 2006 2007 2008 2009 2010Year

Hypothetical Industries, Inc.

Revenue

Adjusted Revenue

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2009 2010

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Hypothetical City Crime

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Principle 3: The chart should minimize graphical complexity

Key concepts

Sometimes a table is

betterData-ink Chartjunk

Generally, the simpler the better…

When a table is better than a chart• For a few data points, a table can do just as well…

$0.00

$50,000.00

$100,000.00

$150,000.00

$200,000.00

$250,000.00

Total Sales by SalespersonSalesperson Total Sales

Peacock $225,763.68

Leverling $201,196.27

Davolio $182,500.09

Fuller $162,503.78

Callahan $123,032.67

King $116,962.99

Dodsworth $75,048.04

Suyama $72,527.63

Buchanan $68,792.25

The table carries more information in less space and is more precise.

The Ultimate Table: The Box Score

• Large amount of information in a very small space

• So why does this work?• Depends on the

reader’s knowledge of the data

The Business Box Score?

• Applying the same concept to our salesforce example.

• How does this help? How could it hurt?

Sales Performance – March 2011

Salesperson TS WD BD NC DOR

Peacock 225 3 40 20 28

Leverling 201 2 45 18 27

Davolio 182 5 38 22 28

Fuller 162 2 22 16 20

Callahan 123 1 15 14 15

King 116 0.5 20 12 18

Dodsworth 75 0.3 12 10 20

Suyama 72 0 8 10 8

Buchanan 68 0 8 8 12

Key:TS – total salesWD – worst dayBD – best dayNC – number of customersDOR – days on the road

Data Ink• The amount of “ink” devoted to data in a chart• Tufte’s Data-Ink ratio:

Should be ~ 1

< 1 = more non-data related ink in graphic

= 1 implies all ink devoted to data

Tufte’s principle:Erase ink whenever possible

Being conscious of data ink

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Hypothetical City Crime

Lower data-ink ratio(worse)

Higher data-ink ratio(better)

What makes a good chart?

020000400006000080000

100000120000140000160000

2011 Total Sales

Order Date

Sum of Extended Price

020000400006000080000

100000120000140000160000

2011 Total Sales

Order Date

Sum of Extended Price

Sometimes it’s really a matter of

preference.

These both minimize data

ink.

Why isn’t a table better here?

3-D Charts

$0.00

$50,000.00

$100,000.00

$150,000.00

$200,000.00

$250,000.00

Total Sales by Salesperson

Evaluate this from a data-ink perspective.How does it affect the clarity of the chart?

Chartjunk: Data Ink “gone wild”

Unnecessary visual clutter that doesn’t provide additional insight

Distraction from the story the chart is supposed to convey

When the data-ink ratio is low, chartjunk is likely to be high

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Example: Moiré effects (Tufte 2009)

Creates illusion of movement

Stands out, in a bad way

$0.00

$50,000.00

$100,000.00

$150,000.00

$200,000.00

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Total Sales by Salesperson

Example: The Grid

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Why are these examples of chartjunk?

What could you do to

remedy it?

Data Ink Working Against Us

Evaluate this chart in terms of Data Ink.

Are there better

visualizations?

Data Ink Working For Us

Evaluate this chart in terms of Data Ink.

Imagine this as a bar chart.

As a table!!

Stacked Bar Charts are Often Trouble

• Original chart from the BBC website

• Why is this so difficult to read?

• What would be a better way to visualize it?

http://j-walkblog.com/index.php?/weblog/posts/bad_charts/

Key Questions: Can you answer…

• What are three aspects of a good graphic?

• How can a chart “lie”?

• What is the Data Ink ratio and how does it relate to Chartjunk?

• When is a table better than a chart?