Graph of a function

So why can't a graph of a function have multiple y values per x, while multiples x's per y seems fine? Wtf is this shit?

>>10
>>6
Wow, helpful replies, I'm shocked.

So while we're on the topic of stupid questions...
I read log-base-b(b) = 1 and log-base-b(1) = 0. What about log-base-1(1)? 1 or 0? Or log-base-i(1)? 0 or 4?