Double Standard

In almost all instances it is a bad idea to place multiple distinct scales on a single plot. Not only does it require more of the reader in terms of deciphering the message, it also lessens the impact of the chart itself. Instead of a reader being able to associate the vertical data with an immediate estimated value in their mind they now have a two step process to determine a value. First, they must choose which scale applies - then they must consult the appropriate scale to derive a value. This is all a bad thing.

I also feel this is a common way to try and hide information in plain sight. Consider the following graph, what points define where the two lines intersect? (Here's a hint - they don't) This layout lends itself to all sorts of slight of hand when talking to or describing data.

Usually, I would claim that any graph that did this could be better provided as either a multiplot or two separate graphs. However, the other day I saw an instance where it made perfect sense: temperature. Plotting temperature with two separate scales is actually a representation of two separate mathematical functions that represent the same property; in effect the graph acts as a lookup table for the user for translating from one function to another. As an example, consider the following:

A glorified lookup table to be sure - but entirely functional.

Moving forward, I've modified my view to something more along the lines of: "Dual scales are useful when they both describe a single property in two distinct ways." Any measurement that can be represented in a variety of ways falls into this category: temperature (Fahrenheit v. Celsius), time (24-hour v. 12-hour), distance (miles v. kilometers), and so on.