That is certainly how we learn it in math, but that's just for convenience (and consistency's) sake. They could turn all those graphs around for the next generation and they'd still learn the same math.
If you compare it to the losing pounds or weight tickers, moving to the right is usually seen as the direction we want to go (a good, or positive thing) and thus left is the direction we don't want to go (a potentially bad, or negative thing). So in the case of maintenance, gaining pounds is moving backwards in the negative direction - at least when it's not just normal fluctuations.
At least that seems to be the basic logic.
| current weight: 185.0