## Seen at Starbucks

##### Dec 25, 2008

Merry Christmas!

If you happened to be in a Starbucks recently, you might have picked up some charts, which was what happened to one of our regular readers and commentators, ZBicyclist, who then tried his hand at chart critique here.  He is worried they may reach millions soon.  So look out!

This graph on the right -- which should rightfully be called a "tree ring graph" -- seems to me a fantastic concept although it is hard to think of data that would deserve this treatment.  Certainly not the retail sales series plotted here!

This is an object lesson in why bubble charts often fail.

One issue is scale: note the awkward way in which the innermost ring is used to designate the oldest sales data of \$375 billion presumably in 1996, and think about how you would decide where to place the 2007 ring.  (It's arbitrary.)

Another problem is labeling: when the growth is slow, the rings are close together, and labels have to be jittered (look at 2001 and 2002).  In this case, a relatively simple solution is to have the entire series of years run diagonally.

Yet another challenge is relative radii versus relative areas.  Inevitably, some readers will respond to the areas while others will respond to radii  ZBicyclist, for example, belongs to the first group while in this case, I find myself siding with the latter.  When the bubbles/rings overlap, it is difficult to assess areas.

Of course, a simple line chart would do the job with minimal fuss.  The following chart issued by the National Retail Federation actually plots the growth rates, rather than the annual sales.

(Please lose the grid-lines, or else add a vertical axis and lose the data labels.  A column or dot chart is also slightly preferred.)

Now go read ZBicyclist's point of view.  In the meantime, let us know if you think of ways to use the tree ring graph.

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These "tree-ring" charts are terrible. I ranted about this form in How to Make a Donut-Pie Combination Chart.

The red outer "ring" with its extension in the lower right reminds me of nothing more than a roll of toilet paper. Probably not the intended message, unless it's to say "Our earnings are in the crapper".

when the growth is slow, the rings are close together, and labels have to be jittered (look at 2001 and 2002)

I was going to complain that there was no need to jitter, just don't label every year. Then I realised that growth had not been uniformly positive, and so every year must be labelled. Which just says to me that only uniformly increasing series are even slightly appropriate for this "tree-ring" format. Sometimes-shrinking series should never be displayed like this.

I said the same thing about the tree ring chart; I guess I'm paying attention. ;)

The "tree-ring" chart, like the much maligned pie chart, depends upon a material analogy for its interpretation. Those who know that the width of tree rings provide an indication of the climatic conditions for those seasons of growth will unerringly interpret such charts correctly. This is not a smarty-pants justification for the "tree-ring" chart (I'm no dendroclimatologist), but an observation that the analogy must be widely known for such illustrations to succeed.

The chart shows us on what happen during year a year,but you can also noticed the high impact on the year 1999 its so high than the other year.Thank you for this chart ring its very informative.

by: alpha

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