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July 20, 2010

Zero-Based Scale Violated

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Today I was catching up on a webinar that Tableau hosted called "Bullet graphs for monitoring and analysis" by Stephen Few. The webinar was quite good, as you would expect from Stephen, but as he was walking through a demo of how Tableau had to implement reference bands in order to include bullet graphs as an option something struck me as odd. Stephen had violated the principle of the zero-based scale.

I just so happened to be re-reading the zero-based scale section in Stephen's book "Show Me the Numbers" yesterday and in the book Stephen says (p. 169):
    "You should generally avoid starting your graph with a value greater than zero, but when you need to provide a close look at small differences between large values, it is appropriate to do so. Make sure you alert your readers that the graph does not give an accurate visual representation of the values so that your readers can adjust their interpretation of the data accordingly."
There are several examples in the book that demonstrate both the effective use and abuse of this principle, but I was quite surprised that Stephen did it in his presentation. Now, I'm sure it was done simply for illustrative purposes, but should he have added a footnote to point this out? Tableau has a nice caption feature that would have worked well for this.

I was able to reproduce Stephen's graph using the Coffee Chain data source provided by Tableau, including using the exact range for the banding that Stephen used. This time, however, I started with a zero-base scale (which is the default in Tableau).

The profits look much more constant in this example. In Stephen's example, the changes month to month are much more exaggerated.

In the end, I'm sure it was done to merely illustrate the feature, but beware of how you use this feature yourself.

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