# The Semantics of the Y Axis ​

The vertical axis is not just important because it embodies one of the most important visual properties, but also because it is much more semantically loaded than the horizontal. Not only does the right choice of mapping help with reading a chart, it can also be confuse people if done wrong.

It's not a coincidence that the vertical is so important for us. An animal that is lying on the ground is dead or sleeping, that's important to know. Vertical movement is also much harder than moving in the other two dimensions, and fast vertical movements can kill us. That is why we overestimate heights: better be scared of a jump that isn't all that dangerous than taking it lightly and getting injured or killed.

We also have some very strong ideas about the vertical direction. Things moving up are generally good, things moving down less so. Being up (standing, walking, moving) is good, being down (lying, sick, dead) is not. We derive many of our metaphors from this fundamental difference too: being down meaning being sad, things looking up or moving up meaning they are good or getting better. Up also means more: more things being stacked or heaped up means more vertical space being used, and more is usually better, so more is up.

## Jawbone UP's Sleep Tracking ​

Jawbone wrote a blog posting about when people slept during the soccer world cup according to the data they were gathering from users of their activity tracker armband. The tracker is called UP, which causes some interesting issues parsing the axis labels in these charts.

Parsing “% of UP wearers asleep” has you going back and forth between two interpretations: UP meaning people being up/awake, but then you read “asleep.” The number is encoded on the vertical axis as more people meaning the line going higher. So more up meaning more people asleep, fewer people being up. I remember some confused tweets from people struggling with this when this made the rounds.

Jawbone also seems to have noticed, since in their recent posting on the Napa earthquake, they flipped the axis to make the semantic connection easier to follow. Now it's “% UP wearers awake,” which makes a lot more sense. More up, more people are awake or, well, up.

The archetype of these visualizations, the New York Times' How Different Groups Spend Their Day also works like this: the bottom-most layer, and thus the baseline of sorts, is sleep. As it should be.