[jpalten]

Hi Robin,

Thanks for the article, it is nice to have some pros and cons together.
I agree that secondary axis can be confusing, dueling for attention, but I don't agree that that is a reason not to implement it. All graphs can be confusing, and it depends on the designer/drawer and the person reading the graph if it is confusing. I can make single-axis graphs that are confusing and dual-axis graphs that are not confusing.

An example where I needed the secondary axis is mortgage payments: it can show the amount paid per year (interest) and the amount due at the end of each year. The interest is a much smaller amount than the amount due.
Yes, you can show this in two graphs, underneath each other, or you can show it in one graph, if you think this is clearer.

There is another reason to want secondary axis: over here mortgage payments are usually displayed that way, so if people are used to view data on a certain subject that way, it helps if you can present it the way they expect. It feels kinda stupid to use a 'duelling' presentation just because people are used to it and expect it, but sometimes there is just no room to be stubborn and present an alternative.