Clarity, honesty, and a sense of purpose

There are two big schools of thinking in terms of data visualization at the moment: there's the ultra-minimalist philosophy espoused by Alberto Cairo and Edward Tufte, where the primary goal of data visualization is to reduce confusion, and then there are those who use data to create beautiful things that uphold design over communication. If you couldn't tell by the title of this section, I generally believe that the former is far more appropriate in most cases. As somebody wishing to visually communicate data, the absolute worst thing you can do is mislead an audience, whether intentionally or not; not only do you lose credibility with your audience once they discover how they've been misled, but you also increase public skepticism over the ability of data to communicate the truth.

Axes and scales are the one of the easiest things to get wrong. You should usually start them at zero, because not doing so can dramatically distort the shape of the chart...