This post is similar to another of mine, but going in a different direction.
The problem: How can we show that a higher number or a lower number is better? For example, suppose we have a percentage, 20%. Perhaps in this case, the higher the percentage, the better. However, in another case, using the same units, a lower percentage is better. What is a simple way to convey this information without changing the data?
Example: 20% of students understand the material (higher is better) vs. 20% of students are consistently late to class (lower is better).
Notes: The solution must fit into a uni-color flat design and should be simple and intuitive, without adding text. I'm looking for a way to show that the number should go up or down, not that it is already moving up or down.