[MertsA]

It definitely is with a tapered cable. The only question is if it could be done practically and if it would actually be worth the hassle. The self support length is for a cylinder of the material, with a cone there isn't a limit to the maximum self support length as the base just gets as wide as necessary to support the rest of it. But with graphene it's close enough that just a 10 to 1 difference in the cross-sectional area between the top of the cable and the bottom would be structurally sufficient. Since the area scales with the square of the radius a 10 to 1 difference is only sqrt(10) larger in diameter between the top and the bottom.

[eloff]

None of that answers whether the structural properties of graphene can be realized at the macroscopic level with such a massive cable. We don't know yet, because nobody has built something even close to that out of graphene.