So, it's great that they try to actually get data on what kinds of charts convey what information. However, you need to know who your audience is. I, for example, found all of their suggested alternatives to be harder to interpret than the dual axis chart. If you're trying to see whether or not the ups and downs of two different variables are similar, suggesting a connection between the two, none of the suggested alternatives do as good a job (although two charts could, if instead of having them side by side you had them one above the other, with the same x-axis scale, but that is really just a stealth dual axis chart).<p>Most of these "don't use this kind of chart" seems to be trying to make it impossible to confuse or mislead your audience, and that is just not plausible. You do, and probably usually should, have some point in mind when you are showing someone else a chart, and the format needs to make it easy to see that. Almost any chart, even pie charts, have some particular use case where they are the best chart for that purpose. No chart is going to always be the best way to present data. Like choosing what kind of language to use in explaining something, you need to know something about who your audience is, and what they are accustomed to.