I attended a web seminar, "Creating More Effective Graphs," taught by Michael O'Connell
and Naomi Robbins and sponsored by Insightful Software, the makers of S-plus.
Michael O'Connell opened the talk with a brief description of Trellis graphics. It's hard to
capture exactly what Trellis graphics are in words, but there is a nice web page by the
developers of this product at Bell Labs
(stat.bell-labs.com/project/trellis).
Dr. O'Connell also mentioned Graphlets. Graphlets are small java programs created by S-plus
to produce highly interactive graphics that you can place on your web page. You can see some
examples at
-
S-plus Graphlets Gallery. Insightful Software. Accessed on 2006-03-24.
(Model, Descriptive) [Excerpt] Here are just a few examples of S-PLUS Graphlets. Some of
the examples include the S source code used to create the Graphlet, which you can use as a
basis for creating your own Graphlets. www.insightful.com/products/graphlets/gallery/graphlet_gallery.asp
Naomi Robbins talked about effective graphs. She is the author of a textbook, Creating
More Effective Graphs, and has a web site,
The choice of a graph form has a tremendous impact on your understanding of the data as
well as your audience's understanding. One graph is more effective than another if the reader
can extract quantitative information more rapidly. You should think about this definition of
"effective" because it is not what most people would use. A commonly held conception is that
a graphic is more effective if it grabs your attention more rapidly. In some contexts (e.g.,
advertising) this might make sense but I believe that Dr. Robbins definition is preferred in
most applications.*
Let me insert a personal comment here: Leland Wilkinson has an interesting take on this.
He splits the types of tasks in a graph into two different categories, pattern recognition
(discovering trends and associations), and table lookup (computing a numeric value from a
graph). Some of the things that help with table lookup (grid lines, for example) will
interfere with pattern recognition.
Dot plots are an improvement over traditional bar charts, and this improvement is even
more apparent when you use a clustered bar chart. By placing the dots side by side on the
same scale, you see patterns more easily than with side by side bars. The panel display,
placing several related graphs in a rectangular grid, is also an improvement over clustered
and/or stacked bar charts. I don't have access to the graphs she used, but I will try to find
or create comparable plots for this and all the other examples described here.
Dr. Robbins talked about a "month plot," also commonly called a cycle plot. It uses the
S-plus command, monthplot. It is very useful way to display time series where you want to
look at patterns across two different time scales. For example, you want to see the trend in
sales over months, but you also are interested in variations in sales across days of the
week.
She showed a polar plot for a library survey where respondents were asked how the library was
doing on a range of scales, what they thought was the minimum acceptable level and what was a
desired level. An important consideration is when the perceived performance was lower than
the minimum acceptable level or higher than the desired level. The polar plot used a cryptic
color system (red means you failed to meet minimum acceptable level on average, for example).
By plotting the minimum and desired values as a range, and placing a dot at the perceived
level, you could easily see when the library was deficient (perceived level was lower than
what was considered minimally acceptable, on average) and when the library was exceeding
expectations (perceived level was higher than the desired level, on average).
She also had a nice display of a 2^4 factorial design. I'm not sure what the name is for the
type of plot she displayed, but I will find an example when I have time.
The choice of color in graphics is important, and you need to keep in mind that some of your
readers are color blind. There is a nice test for color blindness, and it includes a negative
control (an image that even color blind patients can discern). I don't have Dr. Robbins'
image, but I think I found a copy of it on the web (www.geocities.com/heartland/8833/coloreye.html)
Dr. Robbins used these two colors in most of her plots because this choice of contrasting
colors that will work well even for colorblind individuals.
One of the questions at the end of the seminar was about how to best display data that has
a three way interaction. It turns out that Richard Heiberger has a book which discusses
interaction plots in Chapters 12-13. I'm guessing that the title of the book is Statistical
Analysis and Data Display
[BookFinder4U link], which was co-authored with Burt Holland, but I don't have this book
in my library, so I can't say for sure.
[Update, April 27, 2006] I got a nice email from Dr. Robbins. She offered a minor
correction (the title of her book is Creating More Effective Graphs, not Creating Effective
Graphs) which I just fixed and confirmed that the Heiberger book "Statistical Analysis and
Data Display" is indeed the book she was referring to.
This web page was written and was last modified on
09/24/2007.
