Technically it could be whatever, it’s unknowable because the axis aren’t labeled in the original. By convention +y is usually up when graphing lines though. Godot has an X to the right and Y down coordinate system however, as well as apparently html and a lot of image editors

I guess they did show for SpaceY an image of a rocket launching upwards, so that does suggest a Y up coordinate system relative to the screen. And it would normally be assumed that all images would have the same axis orientation