I would not want to be an aging person in a warming world. It's been a long time since I labeled a diagram with an X and Y. So this chart that we're looking at is the level of atmospheric CO2, carbon dioxide. Most line graphs look like a big L, don't they? The x axis is the horizontal one and the y axis is the vertical one. And you can remember that with the rhyming mnemonic x to the left and y to the sky.