Stephen Few of Perceptual Edge writes about primary and secondary axis scales in the March 2008 Visual Business Intelligence Newsletter.
In this month’s essay, entitled Dual-Scaled Axes in Graphs-Are They Ever the Best Solution?, Stephen asks
Is it ever appropriate to include two quantitative scales on a single axis (Y or X) of a graph? Do dual-scaled axes suffer from problems that we should always avoid? If so, is there a better way to display data when it’s useful to compare quantitative variables that have different units of measure (for example, sales revenue in dollars and the number of units sold)? This month’s article attempts to answer these questions.
Stephen follows a rigorous and logical train of thought as he analyzes the use of dual axes in a chart. He examines and disqualifies one use for secondary axes after another, until he finishes with
I certainly cannot conclude, once and for all, that graphs with dual-scaled axes are never useful; only that I cannot think of a situation that warrants them in light of other, better solutions. I invite you to propose viable exceptions, which I will welcome with open arms.
Over the past couple of years I’ve occasionally mulled over this issue myself, in a much less rigorous process than Stephen has followed, and I’ve come to a similar conclusion. Secondary axes are more confusing than enlightening, and the measures people take to clarify them add clutter and chart junk to their charts. While I have written tutorials about secondary axes (and even tertiary axes), more recently I’ve avoided them, and written about better approaches. Rather than making a single chart with secondary axes, it is more suitable to use a panel chart with different scales in the different panels (Panel Charts with Different Scales).
If you have a common time scale, pin the curves to a reference point, to highlight the relative changes of the data since that time (Charting Dynamic Normalized Ranges).
I must admit that I find Excel’s secondary axes very useful. Looking at it more closely, though, I realize that having secondary axes does not necessarily mean there are different scales. I often use secondary axes in order to combine otherwise incompatible chart types into a single chart, as when using an XY chart series to locate labels or lines in a bar or column chart.
An Excel Gantt chart with milestone markers is impossible without using secondary axes.
But in all of these cases, the secondary axis merely adds a compatible scale to the chart for a different chart type.