As with all mathematics, it can be correct as long as you're very careful with what it means. For example, we get this kind of nonsense all the time in Special Relativity, because the interesting physical quantity there isn't (time displacement)2+(space displacement)2 but rather (time displacement)2-(space displacement)2 with the appropriate units. If you can make those units imaginary, then you can replace the minus sign with a plus sign, which a lot of physicists like to do. The picture then illustrates the kind of move that light, or at least things moving at the speed of light in a vacuum, would do. The zero indicates that a clock moving at the speed of light in a vacuum wouldn't indicate any time passing.