x1 is an axis/center curve/extended center (or, possibly metaphorically, focus 'point') of x2 in sense x3
For figures of revolution, x3 = "lo ka ce'u srana lo jbislanu". For rotation (in which mass is actually moving around it), x3 = "lo ka ce'u se carna". For a political axis, x3 would be filled with a reference to political importance or alliance. This word should probably not be used for coordinate systems (coordinate axes), but some construct involving manri might be apt for such an idea if used. The axis is the center of all considerations being made and is extended.