Friday, September 18, 2015

Dual-Axis Charts Can Mislead!

The chart-generating application has scaled these two charts independently, each so it fills its viewport as attractively as possible. However, the sizing of the y-axis scales to fit the windows is not proportional, making comparisons between the charts difficult. (From How to Like with Charts: Investor's Edition, Chapter 11).
A chart with dual vertical axes is useful for comparing two datasets, especially datasets with different units. However, when the two charts are shown side by side, as on a dashboard display, the comparison can be difficult if the y-axis scales of the two charts have not been scaled proportionately. Distortion of results and mistakes in interpretation are likely because typically the chart-generation software has composed the chart for best-fit in the display window rather than for proportional comparison.

The recently released edition of the book, How to Lie with Charts: Investor's Edition, includes the a new Chapter 11: Viewports and Dashboard Displays, which tells how to identify and correct this problem. The price of the new edition has been reduced to $23.99 so it's now the same as the popular second edition, which will eventually be withdrawn from sale. The Kindle price has been reduced to $6.99. Both paperback and ebook versions are available here.

Scaling the Widget chart in the figure above to be proportional to Acme results in slopes that are comparable.

Recently, software vendor Graphician has developed a licensed chart algorithm to solve this problem. The company has implemented its patented algorithm as a plug-in for Microsoft Excel.

In the examples below, without the elongation equalization algorithm, Excel generates by default the chart on the left. The decrease from 100 to -91 and the decrease from 100 to 94 look about the same, but this is a mistaken impression. After incorporating Graphician’s algorithm, the chart on the right correctly shows that the decrease from 100 to -91 is much more than the decrease from 100 to 94.

Patent No: US8,139,065 B2. Patent owned by: Graphician, a brand owned by Chii Ying Co., Ltd.
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[The Graphician example is not a sponsored placement and does not constitute a product endorsement by the owner of this blog.]