I find plotting two correlated but unrelated data series (like temperature and humidity) can be fine. But the article chooses the worst possible dual axis char

I find plotting two correlated but unrelated data series (like temperature and humidity) can be fine. But the article chooses the worst possible dual axis charts where both data series not only measure the same thing (GDP) but share the same units (dollars). What you actually have is a multiple data series chart with actually one axis but you made two to be confusing.