When I was in Israel last week, we wanted to create a visualization that included band lines based on confidence intervals in the database.  In other words, our table contained two measures: value and confidence interval.

Allen Smithee had created some interesting looking Bollinger Bands before, so I started there.  I downloaded the workbook and noticed that while these look decent, they’re not perfect.  This is the initial view:

Looks pretty good.  But when you remove the lines for the upper and lower bands (or if you zoom in), you will see that the bands were made via reference bands for each discrete date, which makes them look like bars.  I suspect the lines for the upper and lower were included to hide the rough edges of the bars.

Here’s how he created the reference lines:

I don’t think this looks polished enough, so I took an alternative approach: area charts.

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):

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.