"Traffic lights in charts are junk. They portray the world in black or white – just using red and green instead. Because they use a bit of threshold voodoo, some people consider them to be an “intellectual” form of chart junk. That makes them even more dangerous – for report authors and consumers alike. And the worst part of it all? The alternatives are so simple.
I used to dislike traffic lights because they multiply the warning signs along the report hierarchies for multi-causal variances. Although this still holds true, I’ll admit that it is a bit complicated. Therefore, I am going to make another attempt to nip traffic-light charts in the bud once and for all. This time, however, I promise to be a bit more pragmatic.
The chart on your left (below) is a great example. You can tell that someone took the time and effort to set thresholds stating when the cells should be colored red, yellow or green.
This color coding is the result of an evaluation process. As readers, we also view it as such. Red signals “bad”, green is “good” and yellow is “okay”.
The report author is responsible for making this appraisal. That fact itself is striking, because the report consumers are actually the people who should be analyzing this information and using it to make decisions. In this case, the report author has already made one decision for them. And just as during downfall of Sachsen LB, we all know how that can go wrong." (Continued via Me, myself and BI) [Usability Resources]