The line #x=0# does not pass the 'vertical line test' since the vertical line #x=0# intersects with it at more than one point (in fact at rather a lot of points). So #x=0# does not define a function.

Explanation:

The equation #x=0# defines a vertical line. It does not define a function of #x#, since there a multiple values of #y# for the value #x=0#.